I get confused with the naming convention used by the Shred Stixx folks. The beaded foam in that gaping wound
is EPS which is NOT Styrofoam. Styrofoam is actually XPS which has huge adhesion issues. Even the current year Shred Stixx claim to be Styrofoam.
Anyway, the way to repair it is pretty much as WAlove described it. I wouldn't buy microballons, clear epoxy and fiberglass all separately, but instead get an epoxy surfboard repair kit.
Foamez carries a good one, with the epoxy, stir stirs, filler, mixing cups, sandpaper and fiberglass. I just went to their site
here it looks like it lists for about $26.
The kit comes with instructions, but principally you'll mix the filler, in this case Q-cell, in with the epoxy to fill that void. Next you'll sand the filled void smooth and finally laminate a few layers of fiberglass over the patch.
I've seen the instructions in that kit and it's really well done. You may be able to find it cheaper on the net, as Foamez does tend to be pricey, although I didn't think the $26 was too bad considering all that is included.
Hope that helps!