For one hour prep, Xanatos put up a wicked defense against Oberon.
Imagine what he could do in a week. I bet he'd have a crack team of Iron gargoyles with cloaking technology walk into The Gathering and just massacre everyone there.
Or maybe send a gift of flowers direct to Oberon that exploded into iron shrapnel.
Give him a year and he'll figure out how to nullify their powers completely.