Cheesy way, neutral monsters don't react to being attacked from 3 hexes away. So get that catapult(s)
True, but I am not sure it really addresses the OP's particular situation (human, early game expansion blocked by greater fire elementals, low research). I mean, it is nice if you have the resources and spells to pull it off, but odds are against it in the early game.I use mostly medium or strong melee units to kill them. Just take endurance on level up, cast an elemental resistance spell (2 if you can) and buy enchanted weapons, silver weapons, or some other perk to help kill them faster. This method requires you to cast healing or a damaging spell, but overall it is pretty simple and doesn't require to many units.