The ALL INVICIBLE army!

[ironballs]

With this setup I get no loss vs. impossible - and very easily - usually 2-3 rounds of combat - as a soothsayer.

Army:
runemages,
archmages,
soothsayers,
demonologist,
black dragon.

Strategy:
first round: summon phoenix spell, call lava golem from rage, and demonologist summon demon - put an entire summoned army on the enemy front.
then use my ranged units (All except of the dragons) to hit from a far - the demonologist use Resurrection if necessary - which is rare,
and area spells (fire rain) easily kills tons of enemies - especially since they're surrounding the summoned creatures.
the black dragon usually stays out of the fight, and mostly use it's special breath attack, or defend the ranged units if somehow an enemy got close.