It'll have to be trial and error, then. At the very least, they're not immune to force damage, and moderately immune to electricity.

My AoO and my first hit have been natural 20s, though I don't know whether these creatures are vulnerable to critical hits.

Confirmation for the AoO: (1d20+24)[43], and the extra damage is: (2d8+24)[28]. This probably means that A5 will have to succeed on a DC 50 Ref save or something to be allowed to move, not sure if that makes that much of a difference :p

However, confirmation for the first iterative attack: (1d20+22)[23], and the extra damage is: (2d8+32)[47].