This only requires:
Level 18
Opponents with 0 MR
Fully stacked Conqueror
Never getting hit by an enemy to turn off Warmogs
EDIT: AND 6 items with a fully stacked mejai's having replaced boots, meaning Swain isn't reaching anyone.
This is actually a pretty cool exercise in sort of triage both for design issues. First you would confirm that it actually can happen (video evidence is great for that) then you look at the severity and likely-hood of it happening to help determine the impact. The severity here could be arguably somewhat high because staying alive permanently in the enemies fountain is a generally game-breaking experience, but then listing out the conditions you mentioned means that, while the output may seem severe. the actual likelihood and impact this would have on the game is very, very minimal or rare, if at all. Doesn't dismiss it, but helps put it in perspective when you are juggling a lot of things!