Worshipper – “Betrayal” [2nd campaign, 1st mission]

At least try playing as a Warrior. I managed to not only defeat Erazial but also kill Rutger there, although I did lose quite a few units in the process. But in general, it all comes down to upgrading your units. Definitely get the boots and focus on logistics. Then, try to clear as much of the map as possible and run away from Godric, trying to confuse him, as he's not very bright. Collect all the Dwelling units, and once you've cleared the map above, you can go into the dungeon. Clear everything there, collect all the dogs, demons, and devils, upgrade them in the Fortress, and you're good to go. You need to try to get lucky and find the "Hellfire" spell early on; then, Erazial won't be a problem.