A bit of a refinement of Luckixxx's method is that an infinite number of red chests will spawn on top of each other. Just continuously go back and forth between the start button and the corner. Each time a new chest will spawn over the previously spawned chest. Then when you have several, go to the chests and open them all. I've done up to 20 red chests at once. Probably about 3-5 legionaries every 5 minutes or so (seems to be more if you have a luck boost). One thing to note is that the more chests that are spawned over each other the more difficult it is to open them. You really need to spam the open button over different areas of the chest. 5-10 chests at once seems to work best and use mayhem 4. If you sell the high value guns at the machines, you can build up cash as well. Once you get to a point where it becomes difficult to open chests, go back to Sanctuary and then return to the cistern and repeat.
A hot fix seems to have been applied around Feb 5th. You can avoid this fix by fully closing the game and going into the console settings and turning off the internet (xbox - systems/settings/network settings/go offline) then loading the game without the hot fixes.