The main rule in the game is to avoid losses, so it's obvious here.

But I still agree with Ment: if neutrals are guarding some useful artifact (say, a cloak with +50% fire damage for a warlock who has at least one of the fire spells), then you can ignore a certain (small, of course) part of your troops to get this cloak. Therefore, in my opinion, there is no definitive answer to this question, unless it is about a battle just for experience. In this case, of course, the losses are not justified.

If we're talking about mages, then summoning is obviously the best option. All sorts of phantoms, elementals, and crystals with walls of swords are clearly not the worst weapons against them.

But what about crystals? Crystals are good for blocking neutral ranged units while melee units reach them. But how can a crystal help against dragons?