I haven't played in years, and gotten to about Ch.17-2, but I recall Titania being ridiculously good for like the first 10 chapters or so. I'm not saying it wasn't the case in easier mode, but it's just that in this mode, specifically, she shines way more, like you actually need her, whereas even HM is still pretty easy when you train Ike and then subsequent characters like Boyd and then Oscar.
Oscar has a really hard time going, if I recall. Enemies take about 4-5 hits to kill, even with WTA (with Ike). Ike barely dodges, let alone Oscar, who will have a harder time getting to double, and even his Hitrate is pretty iffy times (since earlygame is axe-invested). Boyd has to get Str/Spd ASAP to avoid some of the doubles, and even his durability isn't that good.
Pre-promotes are just extra strong for that first part of the game, and Titania still keeps going for a while, since she's getting Exp and doesn't really put it to waste (decent growths).
The Thunder tome 255% Crit. bug actually makes mages far stronger. Even though Soren has some of the worst starting moments, he and Ilyana are probably worth using. If you can get their Atk high enough, they'll likely 1HKO consistently without even getting touched. I recall 1-turning the desert chapter with just Ilyana (unpromoted).
Can't say anything beyond that, other than maybe Lethe and Mordecai also being pretty good (high mobility, solid durablity especially Mordecai).