Originally Posted by Machinimatt
Ok, to get this acheivement do this:
Clear your consoles cashe (removes any temporary game data)
Use a different storage device to create a new save file on
Make all the new weapons and the Laser Sword during case 1-2 in the order listed below (thanks to GuerillaTech)
Make sure the first item you place is the first item listed in the combo, e.g. put the Floor Buffer on first then add the Stun Rod
Don't Skip the weapon combo scenes!
zap 'n shine = floor buffer + stun rod
Lightning Gun = BFG + Stun Rod
Reaper = Scythe + Katana
Impact Blaster = Impact Hammer + BFG
Sterilizer = Chemicals + Syringe Gun
Shocker = Defibrillator + Medical Trays.
Laser Sword = Gems + Flash Light
Laser Gun = Laser Sword + Lightning Gun
I followed this method (with the exception of clearing my console's cashe) exactly and the achievement unlocked for me no problem. Fortunately I was able to use my memory card, meaning I didn't lose any of my saves.
Great guide - thanks!