My sims came out different than yours (7-3 for attacker w/o Unicorn, 9-1 for attacker w/ Unicorn). Without revealing all of my secrets to you (which I did with the Incarnate attack) I will simply say this: Gold medal status is worth MUCH more than 4 stone skins, and especially so in this particular situation.
A Gold medal, Enchanted weapon Iron Maiden has first attack stats of ATT=19, DAM=15 against Evil units. Stone skin isn't going to help the defenders much, whereas a Gold medal gives the IM +1 in all stats plus 2HPs. When Blessed, the DEF=13, RES=15 and HPs=20 serve her well in defending counter-strikes. This is better than any level 4 unit.
So already your sims are set up different than mine and the actual battle. Plus from your results, I can guess a few other specifics that you are setting up different from my sims and the actual battle.
When you say "In our first game my Iron Maiden was killed by a pig and two orc cavalry of yours (it was traveling with unicorn too). Now your one kills TEN quite powerful units" I'm not sure what your point is?
Three Shredders can kill one Air Galley; but one Air Galley can kill 16 Knights. Two Cavs and a Boar can kill one Iron Maiden; but one Iron Maiden can slay an entire army. Three peasants can kill one Incarnate; but one Incarnate can obliterate 3 eight-stacks of Warlords.
Most units are both strong and weak, depending on how you use them. As you just found out, an Iron Maiden in the right situations can be one of the most devastating units in the game. And as in our last game, an ambushed IM can prove to be quite vulnerable.
Former PBEM singles champion.