Something is definitely not right when I got this achievement with 30k speedrun while killing <2000 monsters in total :). Either Bieks is right and requirement needs to be a lot more severe or there is a bug somewhere.

Presumably there was a reason for the original change - some monster types spawning to such a low extent that it was almost impossible to complete, which isn't incredible for a global Steam achievement, perhaps?
Or maybe it's just those cats.
A second achievement could alternatively be added to the roster for near-100% kill rates - Classic Plutonium Man, maybe. Neptunium Man?

There's what, 3-400 non-unique monsters in ADOM? 99% would be really grindy, maybe 98% would be a good goal?

This achievement is almost certainly bugged, IMHO.

[edit]One possibility to make this achievement less grindy would be to allow it to span multiple characters--you get the achievement when you've killed one of every monster, including mutually exclusive ones, but on your account, not on an individual character.

Current problem is that it is a challenge to win and not get this achievement :). There are, I think, 431 monsters in ADOM. If the threshold is set to 1%, it would make plutonium rather difficult but give some slack with regard to ki-rins, eternium golems and djinnies.

Alternatively Plutonium does not have to be tied to winning. As soon as player kills 95% the achievement is unlocked.

I like this plan. It echoes the post in between as well.