How did this end up being about solo creatures again? 5 CR 3s have the same difficulty as 1 CR 11. By DMG guidelines that'd be around 500 hps. Admittedly CR 3s usually don't have defensive CR3 and offensive CR3, but neither do CR11. Point is still that the same difficulty encounter with more than one creature, and especially a far more reasonable 3-6 creatures, could easily result in double the hit points to deal with. Which is of course where AoEs start to shine. Clearly we should be talking about why the elemental monk is the best.