You have to know the grenade launcher glitch, which requires one person elevated and jumping above the other, whilst the person under them has a grenade launcher and which times it for hitting the "sweet spot", launching the individual into the air.

Hold a propane tank while doing this, and then die or get incapacitated from it, and the propane tank will become indestructible, ceasing to blow up until you pick it up and throw it down again.