Helheim is the fourth and last level of the game. To get all tokens you simply have to play through the level from start to finish in either Single or Online Co-op as it states.
It is a long level and to avoid extended play, you are best off choosing a Defender or Bio-Engineer as your class to get through the level without too much difficulty.
You will fight two bosses in this level and neither of them are that hard. There is a dog-like creature, GARM, which you have to kill by destroying all of the separate armoured body parts. Just stay at a distance and keep shooting him dodging the little spike things he shakes at you.
The final boss is Hel and she is split into 3 parts, none of which are particularly challenging. For the first two rooms, just get close and attack her with your sword or whatever you have equipped and then when she teleports shoot her whilst you make your way to her and then repeat the process. The second room is similar to the first. Finally you head up a long walkway and into an arena where she will do exactly the same moves as before. Just do what you were doing before and she will be defeated before you know it.