For this achievement all you need to do is choose the tank class and baa in one of the tree houses in Alvesta. Alvesta can be found by following the map northwest and you will eventually find it, it is a town that is full of tree houses and the tree house you want to use is the one that is the furthest to the right in Alvesta if you go into it and press lb about 5-6 times the achievement will pop.
It is one of the simplest 90G in almost any game, If you would like a guide of how to find Alvesta or a video guide for the whole achievement I have included my guide below