said that he only used the base weapons from the original game, but mine definitely popped after either the Bigun of the Azbats (can't remember which one). It's possible that the game double counts weapons (maybe if you reload a checkpoint, like a lot of other achievements where you can farm kills/disarms/etc.). Just an FYI.
Keep your achievement tracker up, and you can note which weapons you've used. Also keep in mind that modified weapons (like a modified Bastard) works like any Bastard. You don't need special variations. Yes, you need grenade and knife kills. I would not work on this while going for Enlightened, although I haven't finished the playthrough yet, so it still may be possible. If I remember, Enlightened comes easier if you only knock oout rather than kill enemies. (I've gone for kills on a Spartan run.)
I was able to pop this in the mission after I left Sparta at the beginning of D6 in my first playthrough, in the tunnels with the large doors).
At the Church, I switched all my guns to the Azbats, Bigun and Hellbreath. I then got rid of the Azbats when I found a new gun since I didn't want to waste all my shotgun ammo.
Here's a guide to where a lot of the weapons can be found, although this was written for the 360 version: http://metro2033.wikia.com/wiki/Weaponsmith
The weapons that aren't listed are the Azbats (Heavy Automatic Shotgun... burns through ammo), Shambler (the automatic shotgun - 6 shots), Bigun (a pneumatic arrow launcher), and Hellbreath (formerly the Volt Driver).
Whenever you find a new gun, swap it out and use it. If you put down a weapon you like, use it quick and reload the checkpoint (if you can) to get your other one back. I actually didn't use a location guide for this at all, just made sure to switch weapons when I could.
At the beginning of Black Station, Ulman will give you a silenced VSV. As I remember, they're pretty rare. You should be going for the Merciful achievement in this playthrough, so you have 2 options: kill one of the guards at the beginning with the VSV, then reload the checkpoint (the tracker will note that you used the weapon), or hold onto it if you don't have a silenced weapon and want to take out lights.
I don't know if the flamethrower is available anywhere besides the railcar mission (where you should be working on Pyro), but you'll get that kill very easily through natural progression.