CastleStorm

This should unlock for you when you finish the third stage which is the final stage under "prison break". If for some reason you are like me, this didn't pop on the first try. I did notice that one of my Vikings died on the way to the exit the first time I played. This may be what caused the achievement not to pop.

I completed the final stage of the DLC before realizing that I missed this and then went back to it. I played all three levels and this time made sure none of my Vikings died in the third stage. Once the 20th Viking crossed the exit, the stage was over and the achievement popped right away.

If you had this issue or a similar issue on this level, please comment so we can see if it was a weird fluke for me or a common problem for others.

For those still having problems, every time I completed a level and earned an achievement it was on the casual difficulty. I don't know if this is a factor or not, but its worth a try.