I don't think it's that difficult here... Although I failed twice myself.
Basically, you attack the undead with Markel, and the humans with Godric, and elves – whichever you choose. If I'm mixing things up, there are about a hundred skeletons on the right (a bit of a walk), about 18 vampires on the lower left, and 18 liches on the upper left, right next to the castle... I initially went after the titans (on the very first day)... I killed the titans, but lost a lot of units, and then I didn't use my liches efficiently... Markel should have Ice Bolt and Raise Dead. So you can resurrect your own units. The most important thing is to attack everyone with full mana.