Now, to deal with Elwin's army: I would say Stone skin, because of the Black Knights plus this army:
Knights
Paladins
Nothing else
Everything in this army is physical damage based and stone skin will seriously reduce the damage the hero's units would take from all those stacks.
Another possible answer would be Life Light, but it's way too costly.
To make matters a bit more interesting, how about beating this army:
Lake Fairies
Tree Fairies
Dryads
Lake Dragonflies
Fire Dragonflies
Spell to back those up: Stone Skin lvl 3