while i'm definitely no expert in boxing as i've only been at it a few months, i do have the following to add from my own experiences.

at earlier levels i did have some problems where i'd run out of mana on my longer fights or multiple targets in sequential order burning down the power/mana. i had a lot of experience in playing an MT warrior in eq1 so i made the guardian dual wielding and don't use a shield plus i added some procs to his weapons. i then made sure the other team members hacked, smacked, stabbed and crushed when they weren't casting (like noxxy said). this was especially good with the troubador as his melee ain't bad plus the defilers dog is speced to proc ae block and gives haste to group members.

i see you 3 characters which makes it difficult.... if you don't have the dog with the dog buffs yet on your defiler then try that. get all your guys hitting target from behind. let sk take taunt then slow target with defiler +send in dog. heal when you got to and burn with warlock when sk has nice hate. with one healer i don't think i'd want to waste all his mana on nukes.

hope that helps.