Fighting Lion in Destiny 2 is a powerful PvE weapon and is apart of most player's kits. And while it's a Grenade Launcher, it actually goes in the Energy slot, allowing you carry another Heavy Weapon. It's one of the more difficult weapons to find, however, as there is no quest or specific boss that drops the weapon.
It's all about the grind.
Fighting Lion Destiny 2: How to Find the Exotic Grenade Launcher
You can find the weapon through Exotic Engram or Rare world findings, but you could just go visit Xur. The mysterious figure is selling the weapon for the weekend and can be found on the EDZ. It's definitely worth the trek and purchase.
If you have to hold a point or face waves of powerful enemies, with one mini-boss, use the Fighting Lion to take out the boss and switch to your Kinetic weapon.
