It's not impossible with practical optimization at the 6-8 level range, before even getting into TO territory; the fact that it is in fact possible means "It can solo the tarrasque!!" isn't a good measure of power or lack thereof. 20th-level characters are nigh-unto gods; the problem isn't with the PC's power level, it's that a puzzle monster with no ranged attacks, no magic, and no countermeasures aside from bouncing rays isn't anywhere near CR 20.
DR 20 is not nearly as amazing as you seem to think. Heck, a vanilla rogue 20 with the TWF tree and appropriate WBL is getting at minimum 16 damage with sneak attacks, and after other typical feats and items should be hitting a minimum of 25 or 30 damage per attack for 6-7 attacks--and rogues aren't even primary martial damage-dealers.This monk can also take on balors, pit fiends, gargantuan dragons, and titans, and will win pretty much anytime he doesn't roll a 1 on a save or die after failing a SR check, assuming that "they fly/teleport away" isn't counted as a loss for the monk. I really don't think there's anything controversial about saying that with DR/20 this monk is kinda strong, compared to say:
CR 20
Tier 4 classes at level 20
This monk at level 19