I actually ended up doing it the same thing for most of my power exotic catalysts. Load into Leviathan, kill the 8 cabal at the beginning, reload, rinse and repeat. Depending on what weapon you're using, you'll get between 2-5% per run. It takes ~55 seconds to load, plus whatever time it takes to kill the cabal, and they all drop guaranteed power ammo, and all your ammo is there when you load back in.