this one got popular on our server, mostly to attack human castles.
2 rows of myrmidons, which you get from the merc camps (1500 gold, no rare res). they are able to stop cavalry charge.
then elena and 1 or 2 gold troops in the back row to get the required attack rating, which elena will remove with strategic retreat (no losses of rare res).
then jacques, cleo (tome of creation), avalon (summon), virion, (summon), elena (summon) as you like.
the opponent will lose quite a bunch of gold units, risk free.
you can repeat this strategy until most of the defence is gone, resulting in enormous healing times.
to me this feels quite unfair, not sure how to fix it (except increasing the required attack power for siege attacks).