If you are struggling with this Campaign (and you might be if you just finished the other 3 on Standard without maxing your preferred weapons and Classes). I suggest setting it to Casual and just do it that way. I suggest taking a Lancer or Destroyer with Rocket or Grenade launcher as you will be facing excessive amounts of fodder enemies. They also work well enough on the elites when you can just fire at will without having to worry about team damage. The boss fight of the third mission is a very poorly balanced and programmed shit fest so consider using a beneficial card to make it easier and also make sure to never stand in the hugely damaging and overly large vomit puddles the queen and Pathogen Spitters hurl at you for too long.