I remember that argon is a component in certain lasers... and twelve kilograms sounds like far more argon than would be necessary unless the laser itself was massive. Apparently common noble-gas lasers range from several milliwatts to tens of milliwatts (why is "milliwatt" not in firefox spellcheck?? Why isn't "spellcheck"
).
I know very little about laser construction and scaling. I'd actually expect a single large laser to be more efficient than using the same amount of argon in many smaller lasers. However, (if mechwarrior has taught me anything (and if any of that had basis in reality)) then heat dissipation could be an exponential problem with larger lasers. So to be safe, lets split the argon amongst an array of... I want to say 20? Argon being a gas, I have no idea how much it weighs, and doing more research seems like cheating.
So make an array of argon lasers and shine them at someone. Worst case, it's still waaaay more than enough to blind them, but I bet it's plenty of energy to transfer painful and potentially deadly amounts of heat.
Alternatively, since that involves using a ton of stuff that isn't the argon, just release the argon into a room with them. Twelve kilograms sounds like plenty to displace enough O2 to suffocate, or at least debilitate them.
A Connecticut Yankee stuck in the past