Okay, some tips for Co-op.
Kraken: Get the small fry out of the way, and when the boss appears, throw a healing bomb to the boss so any weak party members chaining don't get 2-hit.
Cocoon: Leave the green bombs for when the boss transforms. Do your best to make sure all the small fry (especially the mages who cast the field-wide anti-regen thing) are out of the way. Throw the green bombs in the path of the stampeding transformed boss, and when he runs over them, he gets dazed and his shields go down, making for easy chains until he regains composure and knocks everyone out of the way. When he's not transformed, he'll periodically start a projectile animation. If you're being targeted, start somersaulting out of the way, as this devastating attack can be avoided.
Nibirous: You have to pick up the Silver Arrows from around the field, and use them on the boss to weaken its shield. When its shield is down, you can chain. Not complicated.
If you're inexperienced, you're not useless. Wait until opportune moments to chain. Your chain bonuses greatly increase the damage output of higher level players, so it's critical you get in there and start stacking chains. Also, for these three stages ^^^ don't diffuse magic unless someone is incessantly dropping circles and lagging players. The later dragon boss map is fairly large and diffusing healing circles there isn't uncommon.
Kraken and Nibirous can be solo'd by experienced players, but Cocoon can't and absolutely requires a team to be on the ball. Nibirous also requires a team that isn't picking up Silver Arrows without understanding how to use them. I was in a Cocoon team and while 2 of us had decent damage output, the team didn't understand what the green bombs were for and used them unnecessarily, meaning I had to wait for them to respawn, and we were wasting a ton of the limited time that the boss is transformed. Not even worth waiting for the time to run out.