Destiny 2 - HOW TO GET JOTUNN FUSION RIFLE!
The only place that you can get the Jotunn exotic fusion rifle is the Bergusia forge, which is located in Niobe labs. Niobe Labs is directly south of Outskirts in the EDZ. What sets this forge apart from the others, is that any frame can be completed here!
The other thing that you must have to give a chance at getting Jotunn, is a Radiant Frame from ADA-1. Radiant Frames are what give you powerful gear, and also what gives you a chance at getting Jotunn. Bergusia Forge is great because it can be used to complete any of the radiant frames that are normally restricted to Volundr, Gofannon, or Izanami forges.
MAKE SURE YOU COMPLETE THE RADIANT FRAME AT BERGUSIA!
Hope this helps!