UPDATE: There's a glitch where if you play Orlog and wait until after the coin flip, you can fast travel and the enemies won't see you. You can then go to Werham and get the achievement. I know this works because I personally tested it. Try killing a guard close to the fire to make it more likely you kill him before you burn out.
Video that shows this and the assassinate 10 enemies one working:
(NOTE 1: I take no credit for this video.)NEW UPDATE: Neo Yggdrasill says:
Apparently this wasn't patched in December but it was patched with the 1.1.2 update in February. If you haven't played the game since before the update and therefore haven't updated it you can still go offline first and play on the older patch. I'll try to do this later, I'm still on 1.0.4, I've spent hours on this already back in November and even with this glitch I wasn't able to do it. Hopefully it works this time.
NOTE 2: I'll leave the legit way up in case you want to get it legitimately.
I found this video:
Credit: PowerpyxNOTE 3: You actually need to see the fire debuff above your health bar in order for the kill to count.
According to Powerpyx, you need to be at least 40 levels below. You can drop their health really low, then set yourself on fire. Kill three in one battle while you're on fire to get the achievement. You don't need to worry about the fire going out; as long as you're on fire when you land the killing blow, you should be fine.