This could be the most difficult achievement in the game but with a second controller it is almost a non-issue. Start the game in local multiplayer and select episode 9. This level is the shortest and only takes a little over 4 minutes to complete. Now with the first controller, hold or tape down to have your main profile block the ENTIRE level. Use the second controller to defeat all the enemies. At the end of the level, the player that blocked the whole time will unlock the achievement.
If you don't have a second controller, the shortest and easiest episode is still the ninth. You’ll need to kill 4 enemies right at the beginning and then enter a cage where you have to fight about 10 more enemies. Once that’s done you’ll need to kill 3 more goons and the mission will be completed. This should be done in co-op with the person who wants the achievement as Lori. The best tactic is to stay away from the first fight and once you enter the cage just hide in the bottom left corner, while using your + attack as often as you need to. The other player should play normally with Rick but keep an eye on Lori as an enemy may wander over to her now and then. If you find there are too many enemies surrounding you, use Rick's ground pound attack to clear them out (, , ). Once you are in the final room Lori should retreat to the bottom left corner and Rick can finish the last few goons off.
Another alternative is to attempt it on level 2. Play as Lori, abuse poison and try to always stand in the shadows. The boss in this level dies fairly quick from just spamming the poison ability. Do it in co-op to make it easier. Once you enter the boss room head straight into the shadows behind him (I managed to get by him every time without any problems).