Note: They don't pop till you move. If you need to rest after a kill, just don't move anywhere and you can regen to full.
Note: They will stop spawning after you have killed 8.
Check the deepest floor of Najena's Sanctum
You can access the lower levels by taking the elevator at ( -38, 127, 206 ) Copy/waypoint -37.54, 126.80, 206.36 and going all the way down; right-click the lever at the elevator and click "Lower elevator". It should lead into Najena's Ward of Elements.
Note: You will need to lower the elevator two times, the first stop leads to the level with Najena's Hollow Tower .