What level are you, and/or do you have access to most of the commons and uncommons in your faction?
I ask, because one way to do it is to turtle up and defend, but another way is to send out pairs of units to take out the attuners the opponent sends out. If they don't keep the attuners out, you don't see three or four Worm that Walks. Then, you hang around until you have a bunch of units, and take the enemy home portal. There seems to be a 50/50 chance when you do that, that you won't have to fight the Lord of Despair. If your herald is really low on hp, you may want to consider this route.
Either route, kill all the enemies not at the home portal first. This way, you have less to fight in the showdown.