I actually agree with this, a lot of the times before I did see people in particular upset they couldn't get past like, for example, their 5th streak because of the difficulty ramping up as you went along. It specifically forced you to have a "perfect" team which, for later players it was a boon to keep going. However, this wasn't inclusive to beginner players because they had to "get good" and get the better units.
Yes, it is rather annoying to have to fight easier fights as a more experienced player with at least 4 level 50 golds, but honestly? It's not too much an issue to have an easier streak and work my way up with a really weak or even level 1 fighter because prize fights weren't really "much" on exp to begin with. Even so, the strategy remains the same, the only thing that changes is simply deciding as ImaiKari stated: The efficient route, or, the inefficient route to grind for your exp. You could easily start with low leveled fighters and then work your way up to your favorite, powered up group with one weak character, or go all in with the low leveled fighters and go as you can until you are strong enough to get further up in your prize fights.
That said, I also agree with Zuzuska here. Normally, the way to grind up EXP happens to be the daily character events... But this isn't very much a "good option" as opposed to simply being there to grind emblems / sigils for the characters to use in their nodes. Accursed Experiments is a huge boon to the advanced / expert players as they go along, but it wouldn't hurt to have a weekly / daily event where similar to A.E where the opponent's Fighter Score is bumped up quite a bit to make the maps a good way to grind exp with weaker fighters.
For example, the basic difficulty fighters begin with a fight score of about 1.5k and then the boss is say, 2k
advanced difficulty is around 2- 3k and up and so on with the expert difficulty sitting at a comfy 5k+ difficulty to keep the players wanting to continue grinding. (This scaling isn't accurate of course, this is just my assumption of what would be good to grind up anyone's weaker characters) Later players still get to keep A.E, and beginner players can use both the prize fights as well as the exp grinding mode to get where they need to!