Just make a dummy with a polymorph based ability that costs 0 mana that can target enemies and allies.
also give the dummy -1 hp regen and 1(2) hp.
Events:
Unit is attacked
Conditions:
--------
Actions:
If: owner of attacking unit is an allie of owner of attacked unit
Then: Create 1 polymorph dummy at position of attacking unit
Order last created unit to human sorceress - polymorph attacking unit
Else: Do Nothing
PS: Make sure u base the spell off the sorc polymorph and not the neutral hostile, else it doesnt work