Yeah, I was worried that it would be tedious, so I'm giving up on no-losses for now. In my Impossible Mage game, I decided to do the 20,000 fight for medals straight away, given that I didn't have much to spend my money on due to puny leadership. It's useful given that you will also advance a level, meaning that the Mage can either have Distortion 2 or Order 2. I decided for Order because of Healing and Summon Phoenix.
So the first proper fight either seems to be against gobots etc or scoffer imps. I got the imps this time. It's probably possible to no-loss this fight with just the two droid stacks (I've got the leadership medallion), but the Mage has such low leadership that I only had I think 11 Repair Droids and 7 of the other ones. The enemy hero keeps using Fire Arrow on the droids, and my Phantom spell is only at 25%, meaning that I'm only getting 2 phantomed Droids. Hence I can't seem to keep the droids healthy enough to manage no-losses like this. I suspect it would be easier as a Warrior with higher leadership.
So I bought some comparatively high hp units, Griffins and Alchemists, and did things the straightforward way. Ancient Phoenix is a godsend here and you can use the cursed crosses to take down the enemy (just gotta keep your units of the way - which is hard as the imps cast that enrage ability). Managed to get through this for the loss of 2 Alchemists and 2 Griffins. Possibly I could have chosen Distortion 2 for the increased Phantom power, and then done the droid only path, but I suspect that this won't work in subsequent fights anyway, and you really need summoned units such as the Phoenix.
That's as far as I got so far.
