I really enjoyed this experience. Many battles were really difficult, but I can agree with some of the users that the league was not as difficult as other battles (for example Flannery), mainly because near the endgame you have access to many more resources to beat the game. But I can guarantee that the REAL challenge comes with the Battle Frontier. I just got every gold symbol now, and it was PAINFUL. You can read what follows if you are interested in the matter.
8 reasons why the Battle Frontier is the HARDEST challenge in this game:
1: you don't have ANY ev. It means that your big Salamence sometimes will be slower than things like Glalie, Dragonite and Heracross. Most pokemon will feel really underwhelming because of this, both in dealing damage and withstanding hits. Your best shot probably is using pokemon with really high base stats and good typing (like Salamence, Metagross, Lati@s etc), and abusing their best moves with items like Choice Band for the former two. With perfect ivs and nature, their attack will go up to 504, with good moves like Meteor Mash, Air Slash and Earthquake to hit really hard. Trust me, sometimes this will not be enough.
2: You don't get any access to setup moves (with the exception of Swords Dance, but it costs 48 bp), so trying to cheese your way up to the symbols is not an option. Guess what? Almost every opponent does.
3: Moves like Counter, Mirror Coat, Explosion and Destiny Bond are everywhere, ready to end your run.
4: RNG is CRAZY. Double Team is EVERYWHERE, as well as hax items like Brightpowder, Quick Claw, Focus Band and Scope Lens. But it doesn't end here: I think there is some sort of manipulation of this to the extreme: those items will almost 100% activate when your opponents need them most, for example when you are in ko range of a move and the opposing Slowking has Quick Claw, and the same will happen with secondary effects (freeze etc) and CRITS.
5: Savefiles won't likely save you. I understood that every turn is SCRIPTED depending on which move you're going to use. There is no way to "fish" for a flinch or a crit (or to avoid one) with quickloads. In the Battle Palace it gets even worse, because you can't select your move. You will find that EVERY SINGLE TURN of the match is scripted. The only way to "manipulate" this consist in switching your pokemon (even multiple times) to change the script. I can guarantee it's an absolute nightmare and the only way to get it right is through trial and error with multiple saves. I can simply suggest to save when they ask you if you're ready for your next opponent: if it's not a Frontier Brain, you will always have to deal with a different team, just in case the game decides you can't win. And trust me, in that case nothing can save you.
6: Another annoying but known thing is that the more you advance with your win streak, the more your opponents will counter your lead. I've seen COUNTLESS Ice Beam users (even DUNSPARCE) leading against my Salamence. The game will also know what kind of pokemon can give you more trouble, and will throw it against your face a lot of times. The Battle Tower and the Battle Arena are the worst in this regard.
7: The Frontier Brains are usually not that difficult if you know their team, but there are some exceptions. Spenser (Battle Palace) is a complete nightmare: as I previously said, the entire battle will be scripted from turn 1, so the only way to manipulate it is by switching pokemon. His squad will almost always use the most useful move without losing turns. Arcanine will probably be faster than everything you have (to get over 289 you will need at least a base speed of 115 with 29 or more IVs and nature), and Overheat can almost 2hko a Salamence, while his Intimidate drops your attack and your pokemon sometimes will not move because of the Palace's bs, but Suicune is the absolute worst: sturdy as a diamond, will Calm Mind and click Surf or Blizzard on your whole squad. And guess what? He does have King's Rock, just to get some flinch chances into the scripts.
8: Another nightmare is the Battle Pyramid: because your pokemon have 0 EVs, you will likely be slower than some wild pokemon. Everything will try to defeat you with status spam and bs like Wobbuffet, Explosion spam, Destiny Bond etc, but at least you can prep a bit your team for the challenge because you know what kind of wilds you will encounter (using Gengar was key in the 8th round vs Explosion spam for example).
Other mixed considerations:
Battle Factory is pure fun as you finally have access to actual evs, and you can get your hands on some really, really broken movesets. To complete the run I mainly used Metagross, Salamence, Latias, Breloom, Raikou, Swampert, Gengar, Starmie, Blaziken and Wobbuffet. Without evs they felt really underwhelming, but things like a Choice Banded Meteor Mash/Air Slash (in this ROM it's a 80bp phisical move with flinch chance) are still good. Raikou with Leftovers is really good too, because the combination of his typing, Thunder Wave + Roar really helped against greedy set uppers (for example Anabel with her own Raikou and Snorlax) and wild pokemon like Wobbuffet in the Pyramid. I used Swampert only for my last challenge in the Pyramid vs Brandon because of his Rock Slide and his type advantage vs Zapdos and Moltres, but besides that it reeeaally felt underwelming with no evs. Breloom was fun because of Swords Dance and Spore, it helped me sometimes, while Wobbuffet was simply broken, so I didn't abuse it to get symbols, as using savefiles to use the right move (Counter or Mirror Coat) felt wrong (but you can trust me if I say that the cpu will still find a way to destroy your Wobba by hax or other bs).