For this achievement you're going to need a co-op partner over Xbox Live, if you don't already have one then you can create a gaming session and try to find one here;
http://bit.ly/e6NneJOnce you're set up and have a partner for the achievement, one of you needs to create the match. Go through the menus and select which options you want, to cut out searching for each other set the game to private, doing so will enable the host to invite the other player directly into the game.
Now once you're both in game there's an incredibly easy way to unlock this achievement, just let the enemies pound on you until you've both died and had to continue, doing so will unlock the achievement and prevent you from having to actually complete the game!
For those of you looking to actually complete the game here's some added information!
There are a total of 9 stages you need to complete for this achievement. However some of these I wouldn't consider an actual stage, after every level you'll play a very short "in-between" levels stage, you'll see your character sleeping by a fire and imps will come and raid your potions, this is an opportunity to get some potions by attacking the blue imps and/or some health by attacking the green imps.
Once you've progressed through each level, defeated the final boss and freed the prisoners the achievement will unlock.
I've linked in some videos below which you can use as a guide for each level, they do differ slightly from the XBLA version but not much. Use them learn which enemies to expect and when to expect them and also for tactics on any parts you might be finding difficult. Lastly, while it may appear that I've only linked in videos up to stage 6, these videos will completely cover this game, it's caused by the XBLA title counting the "in-between" levels stages.
* Credit to Madamluna for the videos