It depends on who you want to level up a little. I use the early fights in prize fights to move two people from level 1 -> wherever I get before I need to start using serious teams. The later levels, particularly 40 -> 50 for golds are a lot tougher now. You can use these characters to win the fights in the middle range of 10-16 streaks for some okay gains, then relegate them to the back row of the team when you get to the point where you have to use your best fighters to win consistently. I've been slowly moving up Diva Intervention that way.
Technically, if all you care about is the XP, it probably is best not to go for the highest possible streak, since there's some good opportunity to get XP in that middle range of 10-20 fights. It's just a bit of a bother getting through the 0-10 streak, but you could even use silvers you're working on for that (in the gold prize fight).
I like the Medici prize fights as an opportunity to move bronze characters up really quickly, by pairing them up with some gold characters.
The best way to get xp quickly at this point is in the Accursed Experiments. You'll have to figure out your bare minimum teams for winning those, so that you have room to drag those lower characters along. Bloodbath Eliza is good in a couple of them, since she can't be affected by the bleeds they inflict on her. Parasite Weave and Diva intervention can also do pretty well by stun locking people.
I tend to have three characters that I use in the Daily Events to move them forward. It's not a ton of XP, but it's at least a steady gain every day.
I'm sad that I've finished everything in story mode, because that was also a good way to level up a couple of those characters in that awkward stage of neither being strong enough to carry themselves well in prize fights, but being too high to level up quickly. I guess Accursed Experiments is a good fill in for that now though.
Hope that helps!
