This Achievement is pretty much unavoidable as long as you continue progressing through the story. You will eventually get to a mission called the Farslayer's Apprentice which requires you to defeat two FrostBack Pangars. This enemy is essentially the big bad version of regular Pangar who you should already have experience with. Make sure to take fire weapons and frost armor to have the type advantage and as long as you have a solid squad with decent gear this should be pretty similar to beating Pangar.