This is easily obtained in the first mission you take from the map in the bar when you start the game.
Pressing
on the map should take you to the match making lobby and, if you've not played before, start you on the prologue map 'Tower of Magnus'
There are plenty of opportunities to get this achievement throughout the level, but you will need to find a bomb. These are collectible items often found in chests through the level, and are equipped by pressing
. You will have to compete with your teammates for chests and collectibles, but there are enough around to make sure you have at least one by the end of the level.
I waited until we reached the horn of Magnus. Once the horn is sounded you will be instructed to survive the onslaught. Here, if you face the centre of the tower, away from the wall and the horn, you will see 2 wooden beams either side. Rats will start to descend them and group at the bottom. If you haven't got this achievement by this time, simply aim a bomb at the base of a beam once 4-5 rats are clustered at the bottom.