Really simple way to do this against bots.
Go to Versus > Private > King Of The Hill
Round Score Limit: 240
Number Of Bots: Fill
Bot Difficulty: Casual
Bleed Out Time: 60
Select "Start Game" and then in the map selection lobby, first Y to switch over to the Locust team then ensure you have the Savage Hunter selected as your MP character. Pick any map (I choose Cove but don't think it matters) and start the game. Now the important part - Do NOT pick up frags, inks or incendiaries. You can Bag & Tag with a smoke grenade to get this achievement. Also you never run out of smoke grenades, they keep respawning in your inventory, meaning you don't have to go searching for more grenades.
All you do now is down the bots with your lancer (or gun of choice), pick them up as a meatshield by pressing X and then press up on the D-Pad to Bag & Tag. Rinse and repeat 20 times. I also found that your teams bots are often too dumb to finish off opponents, so I had bots crawling around ready for picking up. I did this on KOTH, mainly so I knew where the hotspots would be, but I assume it would work on other game modes.
Using this method, I managed to get the achievement in about 5 minutes. Good luck!
Below is a video demonstrating this method, thanks to Reabo