The trick is that neutral creatures will only join the enemy elf; they won't serve you. I would do the same in your place. Attack Ilfina and defeat her; the main thing is to kill Talanar and the unicorns, and if possible, the archers (both squads are weak, although dangerous). Then attack with Agrail and the berserker when the enemy no longer has unicorns; he won't be able to counter the spell (hopefully, he doesn't have "Magic Resistance"), and without archers, his chances will decrease even further.