So far I've heard that you can throw items, you can pick up other weapons, you can shoot with a single pistol (I don't remember who thought that was wrong).
What usually screws people up are the miniguns/railguns. You can't use those. Saturday a friend was going for the achievement on No Mercy with another friend and this cool random. We decide to be different and jump onto the roof, throwing down moltovs and gas tanks, with one person on the upper roof holding a sniper rifle. 2nd Horde comes, we can taste victory as we have more than enough health and fire to stop the tank afterward, so I tell my friend with the pistols to jump on the minigun while I and the other with machine guns back him up for maximum firepower. We make it through handily, take out the tank with running, gunning, and fire nice and quick, run for the chopper throwing down a pipe and an extra moltov to cover, and no one gets left behind. But then he didn't get the achievement.
Someone guessed it was the minigun, we went on to do Death Toll on Normal to be sure, and sure enough, that was it.
What usually screws people up are the miniguns/railguns. You can't use those. Saturday a friend was going for the achievement on No Mercy with another friend and this cool random. We decide to be different and jump onto the roof, throwing down moltovs and gas tanks, with one person on the upper roof holding a sniper rifle. 2nd Horde comes, we can taste victory as we have more than enough health and fire to stop the tank afterward, so I tell my friend with the pistols to jump on the minigun while I and the other with machine guns back him up for maximum firepower. We make it through handily, take out the tank with running, gunning, and fire nice and quick, run for the chopper throwing down a pipe and an extra moltov to cover, and no one gets left behind. But then he didn't get the achievement.
Someone guessed it was the minigun, we went on to do Death Toll on Normal to be sure, and sure enough, that was it.