When the monsters began to attack, Zack just used Telekinesis and smashed their bodies agaisnt the ground. Trolls, cyclops, and Gargoyles were supposed to be pretty sturdy, but they fell in front of Zack's might.
The monsters kept being smashed one over the other, and soon, the survivors realized that Zack had them under control, so they didn't waste time trying to turn around and deal with them. They focused on doing what they were already doing since they had that much faith in him.
(You have reached level 270 and unlocked the skill Party System.)