In SF1, the level at which you promote isn't hugely important.
But the longer you wait, the better chance your characters have of ending up with better stats.
Especially Arthur. In my experience, he gets GREAT level ups from 15-20 unpromoted.
And people like Zylo and Bleu always have that chance for a huge attack gain that you might miss.
So, if you're going to level for a while, egress and come back, promote as late as possible.
But if you're just gonna promote as you go without any leveling, just promote at 10-13.
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Congratulations to Rafael Nadal on a Career Golden Slam. - The Mana Sword