ok, so the only mission that you still need to do is justice for the spirit in the deep woods. you need to kill the jotun in the bunker. its just you and borge there, so you mainly need to make sure to attack with borge before britt each turn, and to have valhalla serum with you.
there are no critical hits for your guys in chapter3 (it starts in chapter4).
make sure that you have valhalla serum. in case that you are using burge->britt each round (the order is important), and still miss some damage, if you chose to take fitness in borge's mission to train the boys you can try to redo the chapter and choose the endurance (will) choice. it usually gives you 1 more round.