Leareth has a lot more options than what he's been using. He's not going to reveal Gates even for this (not that a Gate would help especially), and he doubts compulsions would work on the daemons, even if he were willing to show how cheaply he can do enchantment-style magic.
He can go to more complex, high-skill magic than the fireballs and barriers he's been sticking to so far. Which he's hesitant to use because he doesn't know the stability of whatever medium is storing the souls.
(Quick check: is anyone other than neutral evil outsiders in range to watch? No, his Thoughtsensing and mage-sight can't detect anyone within a mile, which means probably he can get away with not having his unusual capabilities spread around - someone could be hiding under a local invisibility spell but that'd be surprising, and he doesn't think avoiding the risk is worth permanently losing human souls.)
He can attempt to trap them in tightly-woven nets of force, it's not the standard paralysis spell for humans since he's unsure that would work either, but it should immobilize them enough to make casting difficult, and be very hard for them to break out–