Hello everyone. I felt the need to do this since unlike Runes, Relics get new additions once in a while, and it'd be a chore to update every single one of my analyses just to add said Relic to my list of options. Instead, I will analyse every respective Relic here and tell in which situations or on which kind of monster they'd be useful on so that you may experiment more freely and I may relieve myself of the need for updating my past analyses.
Before beginning, I must mention that some Relics are obviously weaker than others and some Relics are not useful at all. To more effectively show this, I will list the higher powered Relics before those inferior ones so that you can know the one below is weaker than the one above, but can sometimes be used as a substitute if you don't have access to the latter.
Shields almost exclusively focus on survival, as almost all of them give Armor or Life in certain conditions. They are excellent for reducing the brunt of enemy attacks or Tortures, which is something any kind of monster enjoys.

Tenacity Shield: The most simple Shield, yet one of the most powerful ones. With no activation conditions, Tenacity Shield gives a great amount of Armor to all allies when the battle begins, making it useful in all situations.

Hiroim's Shield: This Shield gives an exceptional amount of Armor even among others, and deals some nice Earth damage back to the attacker. It doesn't protect all allies like Tenacity Shield does, however it can proc more than once, which can make it useful to wear down enemies. It can be used in more offensive strategies.

Fulmen's Shield: Control effects are very common so this Shield will very likely go off at least once. Compared to the Tenacity Shield, it is more situational and may not even give the Armor to all allies, but it has a higher number of procs. This shield can produce exceptional results in matchups where the enemy has lots of control monsters, but note that Control Immune monsters will almost never benefit from it.

Wangzhou's Shield: One of the few Shields to have an offensive effect, Wangzhou's Shield is very much like Hiroim's Shield, but it damages the enemy's Stamina instead of damaging their Life while giving the same Armor amount, but it has a Life threshold of activation. Unlike Hiroim's Shield, however, Wangzhou's Shield has the potential to completely drain what it's damaging, so that makes it the superior Shield in my opinion.

Brave Shield: Brave Shield is something like a hybrid of Fulmen's Shield and Tenacity Shield. It's not as situational as Fulmen yet not as generally useful as Tenacity, but it has the same number of procs as Fulmen. It can also synergize with Double Healing, but be wary of Reverse Healing as it will turn Relic healings into damage.

Revitalizing Shield: Compared to the Brave Shield, Revitalizing Shield procs earlier at 50% compared to the former's 25%, but its effects last for only one turn and it procs 2 times whereas Brave Shield procs 3 times. Most of the time, this is an inferior Brave Shield.

Friendship Shield: This is a less risky Brave Shield with way less reward. It procs at 75%, but procs only once and gives less protection. It gives Armor instead of healing though, which is sometimes noteworthy.

Major Spirits Shield, Major Cold Shield and Major Earth-Shaking Shield: These are, for all intents and purposes, inferior Fulmen's Shields.

Spirits Shield, Cold Shield and Earth-Shaking Shield: Inferior versions of the last three.

Despair Shield: Inferior Friendship Shield.

Last Wish Shield: Almost none of the Relics that require an ally to die are good. This might have been useful in teams with revival skills, but the Armor it gives is so little that it's not worth using.