Try to make a diverse party simply because if you create a party of all melee characters or all magic-based characters, you 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 the 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.