Propane Tank Damage

So I vaguely remember playing this game back in Halo 3 where all the players were stuffed into a tiny room where propane tanks flew at them, and I set out to find a use for the propane tanks flying every which way.

Well, my idea only turned out to be a little different, using an arena instead of a tiny room. But here's where I'm having a problem. Initially I gave the people lighting the propane tanks swords and turned their weapons and melee damage down to 50%. That turned out to be too high and the propane tanks just exploded on contact. I then lowered them to the lowest setting besides 0%, which is 25%, and now it takes two hits from the sword to light them and send them flying, but here's the catch, they don't explode until 15-20 seconds later... This seriously slows down the gameplay as the humans easily dodge the propane tanks while the zombies have nothing to do.

I do know that two shots from the pistol at normal damage will do the trick, but I'd much rather the player hit it. Any suggestions?

 

I wound going with a pistol with 100% weapons damage and 50% melee damage. The map is up by the way: http://www.forgehub.com/forum/reach-mini-game-maps/129944-propulsion.html