The most hellish enema in my life was several years ago.
I was constipated quite hard. And, moreover, there was several days business trip, with departure scheduled tonight. 8-9 hours in a car, on a road with weak infrastructure (2 extremely dirty WCs per 400 km). And long meeting right after that.
I was urge for fast cleanout without additional issues in near future.
The enema was 4 liter of hot (40C, ~104F) soapy (80ml of liquid Castile soap) and salt (120mg) water. I take it in a hot shower which was relatively easy for the first 2.5 liters. Than i stopped the flow for couple of minutes and started again with help of inline pump. When bag was empty i was overbloated. It was not painful but very uncomfortable.
Now it still was okayish... But when i start releasing water it flowed like a fountain with short interruptions. My guts was in agony. It was like something owned my insides via large tube covered with sandpaper. It was hard to breath and fever.
After couple of hours i was so clean that next productive defecation was only after several days. But the cramps was with me for about a day.
Wanna use this recipe for hard punishment, but nobody from my play partners deserve something that hard.