Hello everyone, and welcome to what the title says. This article will focus as much as possible on the roles of Monsters rather than the Monsters themselves, as it aims to be as universal as possible. Without further ado, let us begin.
There are 4 big role divisions in this game: deniers, attackers, supports and tanks, some of which are divided among themselves. Keep in mind that some Monsters can possess elements of multiple of these categories, but usually they will only be able to focus on one due to Rune and movepool limitations.
Deniers: Deniers - officially referred to as Control Monsters - are one of the cornerstones of a team, with their focus being on preventing the enemies from performing actions. An important note to make here is that any denier must, must have an AOE (Area of Effect, meaning a move that targets the enemy team) move that stops the enemy team from moving( "moves that stop (an) enemy/enemies from moving" will henceforth be referred to as "deny moves" for obvious reasons). This is because the primary objective of a denier should be to give its team momentum, and keeping its own team safe from the attacks of enemies - deny moves perform both of these perfectly, and that is the reason why as many enemies as possible should be denied in one move. Also, because of their role, a denier must be as fast as possible, which is why all deniers should have 3 Speed Runes for their rune choice. An exception to this is a secondary denier, which has the qualities of a denier but for reasons that are on a more monster-specific level (lower than ideal Base Speed, has interesting tools for other roles) or due to team requirements do not run 3 Speed - this allows them to deny the enemy team another time after your 3 Speed denier does it, not to mention a secondary denier will almost always have more than just Speed like a 3 Speed denier; they can have Strength or Life runes since they do not to be very speedy to do their job.
Teammates for deniers: Deniers have some natural enemies, like Control Immune monsters or Dodge Area monsters, who throw them off. Look at the enemy team and pick allies that can neuter these enemies who cause trouble for your denier. Aside from this, a denier must almost always be present in every team.
Countering deniers: The most straightforward answer is to just use a faster denier than the enemy's and denying their team before they deny yours, but this depends on a lot of things such as the individual monster, their Ranks and Runes, so I will not dwell on it for long. Usually, a denier has one or two AOE deny moves that use the same deny method, so if you have a monster that is immune to said deny method, you are pretty much safe. Even better are monsters, like Warmasters, that give your team some buffs, and some of them like Warmaster Gortak and Warmaster Thalassa give your team immunity to a certain deny method. Status casters can cast a status at the beginning of the battle, and this can jeopardize deniers. For example, Dunn Ra casts Mega Taunt on herself at the beginning of battle, which draws in all three instances of status and damage the AOE deny would have inflicted, making the AOE deny almost useless, and Warmaster Elvira casts Evasion on her team at the beginning of battle, making almost any move - including AOE deny moves - useless.
Attackers: Attackers are monsters that attempt to kill enemies with damage rather than dealing with them by denying them. More often than not these monsters will have high Base Strength and offensive status effects on their moves like Burn, Bleeding or sometimes even deny effects. Due to them needing firepower, they will usually be running at least 1 Strength Rune, which prevents them from being the fastest monster on the field under normal circumstances. These monsters are divided into 3 categories: extra turn spammers, bruisers and high firepower Monsters. The divide between these is not as clear as the divide between, say, a denier and an attacker, so it is easier to find some attackers possessing elements of multiple categories.
Extra turn spammers: These monsters use damaging moves that, when used, give the user another turn. The movesets of these monsters are designed such that by using these moves in a certain order (this is sometimes not necessary), the monster can use moves for as long as it has Stamina. An example of this is Prince Charmless, who can use 3 of its moves in the following order to achieve this effect: Hat Thrown - Rapier And Hat - Entangled Cloak - Rapier And Hat - Hat Thrown - (...)(These moves are used for this example, they might not be what you need to run on him.)
This type of monster is the monster capable of dishing out the highest amount of damage in "one turn", and as such are the most dangerous attackers to face off against. Some monsters that cannot go on indefinitely but still possess a noticeable number of such moves, like Cain, may also fall under this category.
Teammates for extra turn spammers: These monsters' firepower is determined by how much Stamina they have, so using effects to restore Stamina will greatly increase their efficiency. You can give them Stamina Regeneration, which will give them Stamina every time after they use their extra turn attacks for as long as the buff persists. Giving enemies Elemental Weaknesses is the best way to boost an extra turn spammer's damage output, since direct damage boosts will go away over time. Removing the traits of some monsters on the enemy team, such as Taunt or especially Anticipiation, is crucial for the success of these monsters if monsters with those traits are present.
Countering extra turn spammers: You can deny the extra turn spammer before it moves. However, the worst nightmare of an extra turn spammer is any monster with Anticipation: these monsters were designed specifically to end the era of extra turn spammers which used to be the most powerful type of monster. What Anticipation does is that if an enemy uses an extra turn move, the monster with Anticipation gets an extra turn before the extra turn spammer does, allowing the Anticipation user to stop the extra turn spammer. The more extra turn moves the spammer uses, the more turns the Anticipation monster gets. Another way to counter extra turn spammers is to use a monster that can, through some way, get Taunt or Mega Taunt at the beginning of battle: this will force the extra turn spammer to attack only that monster and potentially miss other, more important targets. Since this type of attacker tends to be as durable as cotton, a faster attacker can make quick work of it. You can also use monsters that give your team immunity to a certain element to stop these monsters.
Bruisers: This attacker type is rare to find, and if you do find them it's likely they also belong to another group of monster. These monsters are best described as "being able to take care of themselves". A great example of a bruiser is Cain, who has good Base Life, Life Steal, and a 0 CD debuff removal that gives him both Stamina and Life Regenerations. These combined make him surprisingly hard to kill, which is a trait that is characteristic of bruisers.
Teammates for bruisers: As said before, bruisers don't need much assistance. However, they welcome commonly appreciated effects, like debuff removals and Trait Disables, to allow them to function better.
Countering bruisers: Positive Effects Block and Nanovirus prevents Regeneration and damage boost effects bruisers tend to have. Buff removal will have similar effects. Taunt and Mega Taunt monsters are able to take hits from them to allow your other allies to hit them hard. If possible, prevent the bruiser from moving at all, and don't rely too much on debuffs to beat them since they will often have ways of removing them.
High firepower monsters: It's safe to say that if an attacker isn't an extra turn spammer, they're a high firepower monster. These monsters rely on very high Base Powered moves and offensive effects like Pierce, damage boosts and Tortures to slam enemies hard, usually aiming for the kill or at least applying great pressure. They are some of the most commonly used monsters because they are the best at being able to take advantage of the momentum granted by deniers.
Teammates for high firepower monsters: These monsters will want debuff removal to get rid of disruptive things like Precision modifiers and Damage modifiers. However, it is important to choose allies based on the enemy team. A high firepower monster will want to strike at the heart of the enemy team by targeting their most valuable monster to kill, and the enemy will have ways of preventing that, so pick allies with useful debuffs like Trait Disable or appropriate deny effects to allow your high firepower monster to do its job without interruption.
Countering high firepower monsters: A faster attacker can kill them before they move if the high firepower monster is frail. If you can, deny them to give your teammates time to deal with them. Taunt Monsters can draw attention to themselves, saving your more valuable teammates from their assault. Some monsters can give your team immunity to certain elements, which can render this type of attacker useless. In fact, these monsters hate any form of damage reduction, so try using them.
Supports: Supports are very desirable members of a team. Their most definitive trait is focusing on all types of buffs and/or debuffs aside from deny effects to help your team or disrupt the enemy team. These monsters are usually the ones to disrupt strategies to pave the way for your attackers or deniers to do their job. They are divided into two categories: buffers and debuffers. Almost all successful supports will be able to perform both of these roles.
Buffers: These Monsters prioritize buffing your monsters with effects like Precision or Regeneration, and usually will have a method of removing debuffs from your allies as well. They may apply very useful effects such as immunities like Phobic Shields, Torture Immunity and Control Immunity, or damage boosts. Buffers are usually the support type to carry moves that give allies extra turns, allowing turn passing to happen. This is a very powerful strategy if Anticipation monsters aren't around, and is extremely flexible in what it can do. Buffers may carry a handful of debuffs as well.
Teammates for buffers: Buffers fit in any team type, since debuffs are extremely common and buffers remove them. Monsters with Positive Effects Protection are prime candidates for teammates to allow the buffs buffers apply to not be removed by enemies. Buffers with specific effects like damage boosts or Phobic Shields should be chosen based on your team's purpose and the enemy team's win condition.
Countering buffers: As said before, specific buffers find themselves more useful in certain matchups, so use monsters that will be affected the least by a specific buffer. Debuffers are good answers in any case to remove any buffs they may try to apply.
Debuffers: Debuffers do the opposite of what buffers do: try to keep buffs off the enemy team and debuffs on them at the same time. They usually carry disruptive effects like Trait Disable, Tortures and Precision modifiers. Some debuffers might also carry turn passing effects, or a handful of buffs.
Teammates for debuffers: Like buffers, debuffers are more useful in certain matchups, and should be chosen according to both teams. Try to find out what the enemy is trying to accomplish, and pick a monster to disrupt that goal that also fits in your team.
Countering debuffers: Buffers are perfect to use here to remove the debuffs they apply. Try using monsters that are not bothered too much by what the enemy debuffer is trying to accomplish. Positive Effect Protection is massively annoying to debuffers since their buff removal will not work as long as PEP is active - it's even worse if PEP is a trait on their target, which is something buff removal cannot remove.
Tanks: Ever since they got Taunt trait, tanks have been under the spotlight. This allows them to crank up their Life all the way with 3 Life Runes and still divert attention from all enemies. What makes tanks so impactful is that unlike other monster types, tanks only need to exist to do their job, because so long as the enemy hits the unintended target, it's fine whatever happens to the tank. As can be inferred from above, tanks are monsters with very high durability that exist to take hits that are intended for other targets. Most tanks will also have access to Mega Taunt, which also draws in all 3 instances of an AOE, preventing enemies from touching your team at all. Tanks usually have moves that aim to disrupt enemies, like deny effects, Tortures and Stamina Drain, to get the most out of the turns that they move on. Older tanks unfortunately don't have Taunt as a trait and will have to set it up manually.
Teammates for tanks: Buffers are really good to use alongside tanks to get rid of pesky debuffs they will inevitably be inflicted with and aid in survivability. Monsters with resurrection skills are especially powerful to revive your tank after the effort your enemy took to kill it. Monsters that use extra turns appreciate tanks because they are able to counter Anticipation monsters, who will have to attack the tank instead of trying to disrupt the extra turn user. An extremely powerful core consists of a tank and at least one monster that has Dodge Area. This makes the Dodge Area monster untouchable, because the tank will take all the single target attacks and the Dodge Area monster is unaffected by AOEs.
Countering tanks: Debuffers are the best way to deal with tanks, to remove their Mega Taunt or disable their Trait (in the case of older tanks, remove the Taunt they set up). The Tortures they apply also help with eating away at their massive Life pool with the percentage damage they deal. A monster with Pierce is the best answer to a tank, because Pierce ignores both Taunt and Mega Taunt to hit the important member the tank was trying to protect. Pierce also happens to be one of the best answers to the Taunt - Dodge Area core.