FINALLY MADE IT !
The one and only trick that was really needed was to get enough tanky units and start with greasy mist+gudridas rage+fire rain. Even when my trolls and black dragons died both in round 2 (dunno how but 6k enemy skeleton archers just one shotted my black drags even under greasy mist :O), I have managed to burn those undead scumbags to ashes with phoenix and paladins under iron skin.
Yeah and...only 30 paladins of my 19k leadership army survived
But hey ! Impossible is possible !