There are two ways to accomplish this. For both, start a co-op game, solo if desired.

Play in Survival mode go on a killing spree. Collect every weapon that is found.

Start a Quest mode map with monster nests (spawns). Have the second player join and survive until there is only one nest remaining. Go on a killing spree at regular intervals, and do not die. Note: Use the Fire Cough perk to dispose of monsters easily without using big guns which can destroy the nest. As long as one player still has the pistol, most kills should spawn weapons, as long as there are not any on screen. Kill as long as possible and eventually blowtorches will spawn.

Note: Nests will eventually stop spawning monsters.