I used the opening mission (A Mysterious Disturbance) for Shadowkeep, which so happens to be the weekly mission this week. Free heavy ammo and a good amount of enemies, with the occasional yellow bar Knights Clear the first area - rinse and repeat. There is also the opening area for the Grasp of Avarice dungeon if you have access to it. Lots of Hive and the 3 yellow bar Wizards respawn fairly quick. The drawback is the chase for heavy ammo when you need it. There is the old fall back of Shuro Chi check point You could also spin up any of the Solo Ops or Fireteam Ops (without matchmaking) , set it to the lowest difficulty and just run those until you get tired of it.