If the combat didn't end quickly, people in the pit were probably going to start drowning, so he let the enemies get close, yelled for everyone in the party to back away, and crunched a "blast globe" (essentially something like a sphere from a Necklace of Fireballs, except glass-like and doing sonic damage) into the chest of an adjacent enemy, killing him and two others and destroying the net.
The fact that he lived through it spoiled the "heroic sacrifce" aspect of it somewhat, but I wasn't going to complain.