Worshipper – “Conquest” [Campaign 2, Mission 3]

I'm having some trouble with this campaign.
At first, everything was fine; I captured one Elven town, then I developed my own town, started developing the Elven town, gathered various items, increased my level, everything was going well. I even lost and regained the Elven town twice, but I can't seem to defeat Gilraen... His army is twice the size of mine... I don't have time to build my army up to that size...