The Jotunn Exotic Fusion Rifle was added to Destiny 2 in its Black Armory DLC, an add-on that also added other exotic weapon for the player to obtain. This Fusion Rifle is considered to be very good, and despite being a fusion rifle, it behaves more like a mini-rocket launcher or a grenade launcher.
If you want to get the Jotunn Exotic Fusion Rifle for yourself, then you have to firstly complete daily bounties from the Black Armory DLC content. Once you have completed 7 bounties, you will get a Ballistics Log. You can get a total of two of these per week from doing the daily bounties.
Once you have the Ballistic Log(s), buy a Powerful Bow Frame for them and use this. Each time you use a Powerful Bow Frame, you have a chance to get the amazing Jotunn Exotic Fusion Rifle.
This pretty much means that you just need a bit of luck instead of grinding to get the Jotunn Exotic Fusion Rifle, and as said above, you have two shots at getting it per week.
Fortunately the drop rate seems pretty decent, so it seems fairly possible that most players will get it within a few weeks, unless they are very unlock.
It does not seem like the Jotunn Exotic Fusion Rifle will drop from any other source just yet, but it's still pretty new, so it could just be that no one has found an alternative way to getting it yet. Anyway, good luck!