Kind of hard to explain so hopefully this won't be too confusing. Go to the area where the cabal are fighting vex and go through the big underground tunnel thing towards the entrance to the dust palace strike. When you get there take a hard right and fight through the cabal in the building complex (up the stairs and keep going). It forms a loop right back to where you started and by the time you go through again, they all respawn. There's always at least one major/ultra and I've seen as many as four at a time. Hope this helps and happy hunting.