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.

The fastest way I was able to figure was to hop from public event to public event to get power ammo, then shoot groups of weak adds until the ammo was depleted. I found that starting EP and running around the thrall there to make them flock up worked pretty well, as you can shoot a big bunch of them with a single WC salvo. Rinse'n repeat.