Walkthroughs for custom maps.

nwc******* will help you defeat any enemy! ;)
In any case, congratulations.
And what are all those black spots on your map? Are you a cheater by any chance? ;)
If you're serious, you can weaken the enemy with a hit-and-run tactic: have a berserker provoke phoenixes and elementals to attack each other and then retreat until only phoenixes remain, and then finish them off in a regular battle. Review the available troops in the occupied cities, assess their total number, strength, and special abilities. And it's very possible that completing some of the unfinished quests will give you something necessary, including for a successful finale. You have a seer in the form of a blue dragon nearby - he's not just there for decoration! If you don't like seer quests, then maps in the style of "kill everyone" might suit you better, whether strategic or linear ("Lord of War," for example).