Magnus is secretly the key to beating level 4. His spell damage is insane and can ramp up to 3k a shockwave, with femur 6k.
Mars can fill a similar role with spell damage with the spear spec.
You want to grab Phalanx ( Bulwark charges spears), Boomstick ( detonates at destination ), and Fire Trail spear (45% dmg per second fire trail)
They can stack and aoe collides on aghanim's model for 5k+ dmg. 10K+ with Femur.