Alexander is arguably the best hero. Use his ability to cause huge destruction to an enemy's base. Aim for towers and trebuchet emplacements.
Make sure to use a distraction unit to draw fire away from Nevsky.
When using Scorched Earth, attack different targets to spread the fire damage around.
With or without his ability turned on, use Nevsky to attack a wall or building in front of a tower. If you do this correctly, he can use area damage to destroy the towers whilst being out of the towers range.