When you’re running a gnoll battle, keep up an irritating nervous giggle during the PCs’ turns. (Did you know hyenas giggle when they’re overexcited or nervous?) Then, during the gnoll’s turns, while you’re busy rolling dice, get the other players involved. Have all the PCs, except the one getting attacked, cackle at the targeted PC. What will that do to the psychology of the person being isolated? What about to everyone else? I can imagine it creating a weird Stanford Prisoner Experiment vibe.
Luckily 4e turns are too short to do lasting psychological harm. PROBABLY!