always take the healing fairy first (green arrow), try to stack tentacle plants (slime + sapling) together for healing, add a couple dmg buffing units to the stack (any imp derivates work).
If possible, make one or two dark magicians. (necromancer + shaman), it's the stronger unit in the game and can easily take out the bosses if properly supported by healing units.
Watch out for this bug: After beating one of the fairies, your troops won't target another one unless you move them closer to the new target. You can easily avoid this bug, just be aware of it.
If you have any more questions, just ask me! :)