Kill waves of ghuls with a captive draug. This doesn't really seem like Cel's style at all, but if that's the way he wants to remember things... The optional goals are to kill 100 ghuls with fire (no problem) and to do it in under the time limit. As always, the time limit is the trickier one, as sometimes a lone ghul will get stuck on a corner somewhere and need to be hunted down. Best bets are to hose the ghul down with flame as they emerge from the ground,and ignite the explosive barrels when possible.
Keep scanning for those yellow diamonds which will indicate strays. In general, don't waste your time on fiery rocks unless there's a distant ghul to take out- stick to regular flame breath to apply consistent, total death to the area. If the ghul are too scattered, 'flee' a few dozen yards, then hose them down as they pursue you. Employ the same tactics on all three infestations for the win.