FOAMSTARS

This is just a grind. You get exp for playing pretty much anything - the only exception seems to be if you play a squad mission and die really early. Keep playing the game and you'll eventually unlock it.

Playing better seems to speed it up - for example, getting awards in competitive modes will provide you with extra exp. The best way to get extra exp, though, is completing challenges. Check your challenge menu and scroll through the different ones. It'll tell you which will give you exp, and usually it's quite a lot.