Gate of Peace
Win E-Class.
Level up your monsters a level or two, if desired, making their levels 11+. You will need two prime attackers. Healer tends to be more and more optional because his HP and Defense are really low. Use him to get DrakSlime, for a Phoenix, to get RainHawk. Check out the Fast Breeds section for more.
Since getting to fight FangSlime is a little hard for beginners, here's what you do. Play all 4 slot machines. If you have trouble playing the one FangSlime is at, stand behind or beside it and press A. Talk to FangSlime afterwards to fight.
FangSlime is a bit harder than the other bosses, but you shouldn't have too much trouble. He has CallHelp, which will do some damage. I haven't seen him use it much, though. ChargeUp will do fairly high damage, so you may want to defend when FangSlime is charging its attack. WarCry will immobilize your monsters, almost like a sleep attack.
FangSlime will join you. He may help a little on the offense side of your team. Breed him after a while, maybe with a Beast to get Unicorn.
Next