hm, thats very weird. ( and obviously not supposed to happen )
I'll check this right away ( scene.bin fixes are easy to do )
do you have any other mods installed? becuase other than the name, Gamma uses the exact same data as the vanilla Trine
do you have any other gameplay mods installed?
To narrow this down, try saving before him, make a backup of your scene.bin ( in ff7->data->battle )(scene.bin.backup) and then put the vanilla scene.bin there and see if it still crashes.
what attacks specifically have you seen that do NOT crash?
also, do you have the latest core files? ( see page 1 of this thread )