1) Aventurine (honey timer, the need to refresh tauros/bouffalant)
2) Hardy's gym (awesome theme, but horrible)
3) Mirage Tower (fully supportive of this level of annoying)
4) Scrapyard (endurance)
5) Totodile (not so much hard as pissed me off)
But that's just me, probably because I made myself do all of those for pride - I'd never have worked out Beldum, Jangmo-o and others without wussing them out and looking them up. Very keen chess player myself, so Radomus was just awesome. To be honest, I'm just glad (astonished/nervously anticipating) that Ame hasn't replicated the water currents in RSE, there's more potential places to use that narratively speaking than the portal mazes in Devon Corp. Still remember the frustration flashbacks over being too cack-minded to find the Regi diving spot, although I still contend that the whole puzzle is THE most Reborn-worthy from the main series games, with Wailord and Relicanth and HMs et al.
Hang on. If you're surfing in a cave with level 100 pokemon (say... Victory Road) then that'd be a good way to punish missteps, repel's not saving you. Particularly if they're fast enough to whittle your team. Like Starmie. Or Crobat. Or Sharpedo. Combine that with the need to use all the HMs. Or league officials who take away your bag beforehand like in the RSE Battle Pyramid.
I'm CERTAIN, however, that Reborn's Victory Road will be more faultlessly brutal than I can theorycraft; I'd like that - I mean, it's Victory fecking Road, epic is mandatory. Hell, I'd like it to be as hard and as long as all the worst puzzles COMBINED. Maybe it'll have been designed by Lin - after all, she's the one that wants us in agony and yet also wants us to thread the needle.