Not Shelly's Gym Puzzle? I hear everyone found that to be the True Final Boss.
Remember that this is my fully subjective opinion.
Under the assumption you do not exceed level caps (for evolution, or just haxxing win)
Corey would be a good qualifier... however, the real question here is whether or not most people will go through the trouble of raising a team to deal with a gym they (assuming first play-through) know nothing about, including the type. For that matter, unless the person in question is big on sidequests, they'll probably get the ever-so-fun corrosion field.
But hey, they could have one of those lovely Ground- or Psychic- types with them. Or a starter that evolves into one, for that matter.
Poison and Steel are also good, and there are confessedly a lot of decent Poison types available. Steel, however...
I'd say it's reasonable to assume the "average" person will have one Poison-type and one Ground- or Psychic-type with them at this time. Since Reborn pushed for diversity, let's assume a full team of six Pokemon. Level-wise, I would expect everything to fall below the 26-27 mark, and, if they are using a mystery egg Pokemon, it be about 20-22 (again, assuming minimal grinding). With such a poorly-defined, imaginary team, and no clue what to expect (aka the entry hazard), nor a reliable counter to Corey's Crobat team (due to not having the location guide on-hand), it is reasonable to consider the number of attempts for Corey to be an average minimum of 2-3, with the maximum being... Rest in peace, any Fairy mono people who opted out of C-Mist.
If I had to give any "main" reason for Corey being difficult (personally or otherwise), it'd be the combination of 2x Poison SR damage, Venoshock always being 130BP, and Crobat outspeeding everything you have with... reliable enough STAB attacks.
Actual spoilers in the spoiler (but still another reason)
Just to say it again: ^All of that^ is my opinion. Feel free to disagree.