The "Summon Phantom" spell is given as a bonus in the first or second mission. It remains available until the end of the campaign, even without summoning magic. I played this campaign in version 1.3 because version 1.4 crashed, and 1.41 wasn't available yet. Therefore, there was no particular reason to return for more units – in version 1.3, creatures don't accumulate in dwellings. I had 128 dragons (4 stacks of 32). I defeated him on the second attempt. Agrael was level 30 with a super ability. I summoned phantom demonesses.