I wouldn't have the fighter move in after the piosioning. I'd have him positioned nearby first and then moving away before setting fire to everything. That'll let you poison + burn + get a hit in. Could also try a mate and archer combo instead or multiple mages. That way you don't have to worry about coordinating.
Found the poisoned food by Butter but pretty sure I've seen more poisoned items and stuff around.
Edit: again, the above assumes a zombie fighter. It's just too hard to coordinate what your trying with a z movie mags and non zombie tank
Believe you me, I am well aware. But while the poison/fire would do lots of damage, what do you reccomend I do if I want to heal someone? If I'm using my poison offensively (which, I think, is the best way to use it), then what do I do when someone gets low?