Level everything to 50, there are usually enough eggs and exp per event to do this. I have every event character at least to 50 and some of the dungeon characters too.
Realm Synergy will take care of some of the missing stats to 65, so you'll have more "high level" characters for each realm and you'll be able to hit mastery conditions. You'll have a few like Cloud hit 65 anyway because you use them all the time.
I'm currently using two White Mages in Retaliate Meta for Shellaga/Protectaga/Boost/Curaja.
If I needed Shellaga and Protectaga, I'd level up P. Cecil because stats. So far, I've gotten away with one or the other; judicious use of Breakdowns and SG RW have provided me enough mitigation. If you're going all-in reta meta, the marginal loss in stats (DEF, RES, HP) I can see the preference, but I've stayed away from that.
It's the characters I use all the time that have been neglected in levels, ironically. Quistis, my main support/secondary BLM, was just barely into the 50's whereas now she's 62 - huuuge stat increase to my team 95% of the time; Vivi, my only BLM, was just 51.
I want to level Irvine and Balthier because they have good RM's but will sit on the bench 99% of the time (c'moooon thief abilities!) unless I'm lucky enough to land a good gun during SB celebration. But even with RS, Snow, Kimahri, Steiner, Tifa, Kain, Cyan, Edgar, Sabin, Refia, Luneth, Josef, Ricard, Gordon, and Maria aren't worth it over strong characters like Cloud, Squall, Lightning, or Sephiroth. We have too many characters that are absolute shit - they have no redeeming qualities. Once we get thief and knight abilities, we'll have some more options and strats to explore but in the meantime fuckem. Worthless string of characters.