Hello,
Till now the best farming place for exp is Kingdom of the Sands – Powerslave battle, if you want to get up your troopers just get Edi and one trooper and you will get 800+exp/battle

Healer for what I know is the allied soldier has recover = magic + max hp. I have the undead allied soldier heal all = magic

Fighter I have - Soulless demon

Mage I have - Child of the damned

Gunner I have - Allied Bomber boy

Assassin don’t have yet.

Please tell me your opinions about the best troopers in-game but not Edi please.