The monsters are grouped in groups of 2 or 3 and sometimes there's one patrolling. You'll make it a lot easier if you can pull the patrolling one without the stationary groups. It could come in handy to bring an ability like bubble trap to temprarily decrease the number of monsters actively fighting your party.
The first boss you'll encounter is quite easy, clear the room a bit to give yourself some space. You don't need to clear the whole room. The boss wil drop an increasing number of barrels wich explode after a while. Other than that, he jumps around at random targets. As long as the tank has aggro he should return to the tank. The healer should keep everyone healthy as the boss can pull everyone in and cast an aoe, the aoe isn't that strong though.
The second boss is the hardest one. This boss doesnt tolerate a lot of mistakes, if your party fails to avoid unnecessary damage your healer is going to have a rough time. The boss will summon sparks/lights that explode after a few seconds, the sparks spawn on each member of the party,except the tank. After the boss has taken some damage it'll start summoning meteors that spawn 2 small elementals. You should destroy these as fast as possible. Beware when they die, they explode and deal massive damage. If you dodge everything correctly your healer should only be healing the tank, except for a few spells on party members that got caught by the random meteors.
The third boss is the easiest one. When the boss is around 60% health, he will start casting a spell that will 100% kill you. The only way to avoid death here is to stand in the circle cast by the friendly NPC, wich spawns at random locations in the area between the 4 pllars. The boss will also spawn adds which drops a red ball, nearby party members will receive full mana/health and extreme casting speed when the ball disappears.