The achievement is maybe not glitched, I saw different lists and there seems to have a misunderstood.
My brother wasn't able to get the achievement and we just noticed he was missing the "Laser Gun". We just did it and he got the achievement.
I don't think the order you use for the weapons or neither the fact to skip the cutscenes have effect on them.
Most of the list aren't very clear about the "Laser Sword", Laser Gun" and "Lightning Gun"
Here's a list which should guarranty to give you the achievement :
- Zap N' Shine : Floor Buffer + Electric Prod
- Sterilizer : Chemicals + Syringe Gun
- Shocker : Defibrillators + Medical Tray
- Impact Blaster : Impact Hammer + Blast Frequency Gun
- Reaper : Sickle + Katana Sword
- Lightning Gun = BFG + Stun Rod
- Laser Sword = Gems + Flash Light
and the last one, the "Laser Gun", wich is a combination of two custom weapons created before, the "Laser Sword" and the "Lightning Gun".
There are eight weapons, and not seven.
The achievement guide list the eight weapons, but the last one is mixed in the sentence, so it's not very clear
Gems + Flashlight = Laser Sword combined with a Lightning Gun.
Most people will ready that as "one weapon" when it's, in fact, two.
Hope it'll help some peoples.