The so-called beast dungeon turns out to be a dinosaur paradise. Before Cain can summon his forces, he is beset by a horde of tiny carnivorous lizards. They're bipedal and less than knee-high, so Cain calls [Acid Rain] to blanket the area, burning and melting their skin.
Only those closest to the edge of the spell manage to flee, but the screams of those who don't make it out in time draw an increasing number of reptilian attackers to the dungeon entrance.
Since he's already merged with Inquisitors, Cain transforms into Ancient Seraphim form and uses [Ancient Resistance] to triple the numbers of the Seraphim army he calls to counter the giant lizards.
Spears of holy light immediately slam down on the attackers from above as the Seraphim go on the offensive, pushing the dinosaur-looking reptiles back enough to make room for their hosts.