Leo and Corrin don't look worth it to me, imo. Both are entirely subpar to other top-tier mages of their color, and Corrin might only be useful on a fully dedicated Flier team. The problem with that is that Flier Emblem's biggest weakness (besides Bows, I guess) is that both of the Red Fliers aren't really that good. That makes Green units the biggest threat... which Corrin is weak to. Add in the fact that you'll have to give her a Blarblade to be useful, that she's literally just a weaker Delthea on non-flier teams, and that she can't be merged once the event ends, and Corrin's just not worth it unless you really *really* need a blue mage or something.
Elise looks very good, though I haven't done any calcs on how she compares to the others. Xander also doesn't look very strong, especially competing with the likes of Hector, Minerva, and Cherche. However, Infantry Pulse is a brilliant skill. It doesn't help Flier/Horse/Armor emblem, it gives something to Dragon emblem, Bladetome teams can't entirely abuse it due to Hone and Fortify skills taking up C-slots on those teams, but other infantry teams get a massive buff.
Basically what I'm saying is to summon as many greens as humanly possible, to sacrifice Xanders to literally every Infantry unit, and because Elise is in the running for top-tier green mage.
EDIT: Elise looks to be better than Nino if they're both unmerged with Gronnblade and team support. At around +3 Nino pulls ahead of an unmerged Elise, and when both are at +10, Nino is better. Elise does well with Life and Death and Desperation.