It seems this isn't the most optimal option. I attacked with a Hero + Imp (the Imp drains mana from the Leprechauns), but I couldn't take on more because I needed to escape from more than two Leprechaun units. I used magic to drain the Leprechauns' health, and the Hero can withstand a couple of hits from them. I infected one unit each and kept running. I cast 'Despair' on both units beforehand to avoid having to deal with morale and load/save games – I'm tired of that, and what's the point of all the different magic in Heroes if not to be used? :)
Not bad... so, this way, you save a bit more mana? I hadn't considered this option because escaping from two Leprechaun units... well, it's inconvenient... and besides, your health will be almost at zero, so you'll need to return to the town, otherwise you won't be able to defeat the Fae.
