I have experienced many of the things you have in the game also. I am
current a level 14 wizard and I am slowly learning to conserve my manna
and to keep the monsters off me while grouped. What i do is generally
stand just in range of the monster and hit it hard. Then I fade back
and let the tanks take it. After the have the mob securely contained, I
watch the fight as it progresses. If it's going easily for the tanks, i
may not even cast again. If the going is tough, I will take the minster
down as fast as I can. If we are fighting more than one, I try to
monitor the health of all the involved monsters and take down the
weakest first. A weak monster ca still hurt someone nearly as well as
a strong one. Once it's gone, I check for the next weakest or the one
that's hurting us the most. and flame it next. With any luck, we all
live the mob dies and I still have a couple of bubbles of manna left and
no loss of hit points. Of course sometimes I mess up and hit something
too hard and they start trashing me but usually a good group can pull
and taunt the mob off of me. If not there's always shadow step and then
root.

Dupont human Wizard of Freeport
Espirit de Corps Order of the Head
Cazic Thule