Enter Forge mode on any level where you can place both fusion coils and propane tanks (High Ground and Last Resort recommended). Place as many fusion coils as possible and turn their run time minimum to the amount that you have placed. Do the same with the propane tanks. Make sure they are within close range of each other. If you shoot a fusion coil or propane tank, the explosion will set off all other propane tanks. When the other explosives respawn, the propane tanks that are currently flying around will usually detonate the fusion coils that have newly spawned.