Try to make a diverse party simply becuse if you create a party of all melee characters or all magic-based characters will eventually run into huge problems with some types of enemies. It's the same as with all RPGs.
Keep in mind that healing requires Action Points to use so if your character's HP is low be sure to heal him before you decide to attack enemies.
Again as in all MMORPGs communication is key. Use game's Message feature to let your party members know what you’re thinking, what you want them to do and/or to call their attention to potential problems.