im very new, but I got the cat stith medic monster at the Bresha Ruins and leveled her to level 9. I don't want to level her too much or infuse her with others monsters (never done this yet) before knowing if there are any other better monsters.
I just completed the bresha ruins and the flan tomatoe bosses with snow in that jungle area plus got the artifact from the Colliseum, which was a strange area. I guess I beat it by just talking to that thing?
I just want to start leveling up and infusing a godly monster, which I prefer is a healer.
Also, how do I know which areas are a part of the main quest?
Cait Sith is actually a pretty solid healer. It'll easily be good enough to see you through the story. You could also level it up to 21 so it learns Cura and then infuse it into a Spiranthes, which has an ability that increases rare item drops after battle that comes in handy.
If you want one of the real top tier healers, wait until you reach Augusta Tower 200 AF and capture yourself a Flanitor. Along with the Green and White Chocobos it's one of the top three healing monsters in the game, and much easier to level up than the other two.
Monkey see, monkey do - just another animal in the zoo.
However, you're fine with Noel and Serah as healers. Serah is a really fast caster and Noel has group healing spells. I also found potions to be more useful, than in the first game. Get the item that doubles the potions effect and you have a cheap and fast way to heal. Potions are very easy to receive, as they are very common monster drops and Chocolina sells them everywhere.