I wonder if I was the only one who had a problem with CYnthia's Milotic in Platinum that loved to do Mirror Coat.
I found Sabrinas Espeon quite tough in HG/SS as its got so much power and its not that long after the elite four so you would not of had much of a chance to level up yet.
also in HG/SS the level increase between Pryce and Clair is so drastic that you may be underleveled when you play her as there is not much grinding opportunities between her unless you fight 100s of wild pokemon lol.
Bryce strongest was 34 and clairs was 41.
then the level increase between Claire and Elite 4 is drastic again.
since victory road has weak type pokemon XP wise it takes a lot of effort to be around level 50 for Lance. (That was one of the more frustrating elements of HG/SS the level spikes in short periods of time between major NPCs)
I agree. There's a similar situation in the original RBY games, where after Erika there's three different paths you can take, so they're all the same level, but whichever one you take first will be difficult since you'll be under-leveled no matter what. At least in GSC where there were the split paths they made it so both were a natural progression from gym 4, although that results in whichever bit you do last becoming very easy. In Yellow the way they did it meant that there was a huge gap between gym 4 (L.32 Gloom) and gym 5 (L.50 Venomoth), even though really you can get from gym 4 to gym 5 just by going down cycling road, which has only about 20 trainers on it. You don't level up proportionally, although there are plenty of good places to train at least.
HGSS actually shortened the level gap between Pryce and Claire, from 31->40 to 34->41. I agree Claire is very difficult. I'd say moreso in HGSS than in the originals. I usually find the Gyarados the biggest problem, since it's got such a high special defense and Intimidate. I don't usually have anything with an electric move (especially when I'm doing a non-TM run). The Kingdra is a challenge as well, but I find it easier to kill.
After Claire in HGSS you have the Kimono Girls as well, who I also find very difficult sometimes. The Umbreon in particular can be so difficult to wear down if you don't have Leech Seed or PoisonPowder or something. It would be a lot easier if you could heal between them.
I don't think the level increase between Claire and the E4 is that big, if you compare Claire's levels to Will's, and at least there's a good amount of trainers inbetween, but the HGSS E4 is difficult for sure. Will is difficult, especially the Slowbro, also Bruno's Machamp I sometimes struggle against. The worst is probably Karen's Houndoom. In a lot of cases just one Nasty Plot will allow it to outspeed and OHKO my entire team, and all its moves have 100% accuracy. At least with Lance a lot of his moves can miss which gives you a chance to retaliate or heal.