I have copied this strategy by TheBigToll from the Strategy Guide section on its page to this thread and recorded a link in its place.
Strategy 1. Poison 6, sniper 5, WoS 1, slow 6, clarion call 4, Sapphira 4, Valorius 4, Furnace 2 ( u can use any high lvl hero). Place slow on the far left place and poison on the next one, WoS on any other place next to it. Summon all Heroes + clarion call between WoS and poison. Upgrade poison both abilities to lvl 1. Then place snipers. You will need Boots of Quantum Stride to complete this level easier.