That, to me, is part of the fun of it. The Riddler has SO loaded the bases that it's all going to come crashing down on him, metaphorically speaking.
The knives is a good point, I wondered whether Robin is actually tied to a winch, so he'll be being rotated round and round to make the knife throwing more of a challenge.
So the electric shocks will set off the piranha, who will attack the shark, and it'll thrash around a bit making Riddler miss with the knife, accidentally cutting the ropes securing the Teen Hostage (Brunch Wonder is one I'll have to remember though! :) ) who will manage to knock the Riddler out for long enough to use some of the acid to dissolve the chains around his ankles... and so on...