Let classes choose their OWN cross class skills (level sync woes)

[Felis]

I'm confused. Nobody can use Stoneskin below the intended level if you sync (that includes any cross class abilities you have equipped), which is the problem you're having. Yes, any class can use it at lv.1, but they're not synched either.

Level-sync only reduce the number of the unlocked cross-skill slots, not what you can put in.
You can as example use Stoneskin in the level 15 dungeon if you put the skill in cross-skill slot #1.