Play in Co-op mode. In the last area of Thames when you get the tank, take it through to large snow pile before you lose it. Leave one player in the tank and have the other player out of range of the Steelheads and the Titan. Let them destroy the tank and allow the person to respawn. They will be invisible and unnoticed by enemies. You cannot die unless you jump over an edge. This glitch only works until the end of the level.