@death-march : I think you have great monsters for someone playing for only 3 weeks.
I have been playing for four years and I still struggles with PvP, so welcome to the club
Patiense and experience are important. Even if you spend a mountain of gems on monsters, relics, runes etc - you still have to learn to know them.
During battles you learn not just about your own monsters, but also the opponents and that experience you can use for selecting a defense and attack teams in PvP or elsewhere.
In general legendarys are better than Epics, but not always: there are excellent Epic killermachines like Volray and Nautilus and also numerous excellent support epics. R's and C's are not so good, but they are easy to get and to rank up (for instance the R Pandaken that so many of us breed - in the beginning for gold, a little later to level up and as support (with Team runes). As you probably know Pandakens are excellent gold producers!
I imagine at this stage you need food and gold to feed your monsters and progress through the game. So use Monsterwood and the Dungeons when you can.
But monsters and levels are not all. Runes are very important, so when you can: activate and level up the rune building. Level 1 runes cost "only" gold (I know: in the beginning you really cant get enough gold). level 2, 3 and IV runes can be crafted with ease from here on.
Use Dungeons and Monsterwood to get extra gold, food, relics and runes. Also pay attention to the news: for instance runecrafting with almost no timedelay for level 1,2 and 3 runes and 50% time reduction for all others.
Legendaries are on offer all the time, but especially Team events (Team Race, Battleground etc) can help you get them or at least be helpful in building your account.
Personally I would say: avoid gambling. You can get lucky, but - as with all gambling - you most likely will not! Gambling elements is designed to trick players into more gambling and thereby loose more money. Some like that kind of thrill, though.
Breeding is another way of getting fine monsters. This post show how to breed Legendaries:
Getting on active team is also very important. You get team mates, can ask questions, you can exchange monster cells and in Team Wars you can earn war - coins you can use in the team shop to purchase monsters and, at a higher price: runes. Find a team that is loyal to you and stay loyal to them. If you, for whatever reason, has to join another team, tell the lead in good time, so they have time to prepare. Same thing if you cannot play for a while.
My "advice" here is only rudimentary - others might focus on other aspects, but anyway: look through the info on these pages, there is really a lot of good help - from very helpful players - you can benefit from!