Well, I dunno how far along the story you’re in, and I dunno if you’ve completed Chain Challenges, Squad Assault, etc.
If you’re wanting to focus on Arena then you’re going to want highly merged units with high BST. Of the units you listed so far, Robin is the only one that’s fairly easy to merge, but I wouldn’t really recommend him unless you’re using B Duel Infantry.
So excluding Arena (simply because the units aren’t that good or too hard to merge), we have Story Mode.
Story mode, and most “PvE” content are easy if you have the right setups with the exception of Abyssals and a few maps here and there.
Alm and Celica are some good red picks, Alm especially because he can deal true damage with his A skill.
He’s also gaining an additional +5 to all stats thanks to his prf weapon. I’d probably run Desperation as his B skill and something like Darting Blow/Swift Sparrow/Spd Res Solo in his seal.
Her new refine has given her quite a bit to work with since she gains +8 to all stats with it completely active. She can be offensive and/or defensive and do it fairly effectively. Her build depends on what you want her to do, and what fodder you have for her.
She can probably run a fairly decent Galeforce build.
Just be aware of Guard effects.
I wouldn’t really recommend using Amelia unless you have a unit that can actually help her as far as traversing the map. A Guidance flier or another Armored unit to help proc Armored March would be ideal (unless you’re running a full team of Ward Armor
like the good ol’ days). It also doesn’t help that she’s got a speed band since speed is one of her more important stats. Luckily she’s gaining +5 speed her Grado Axe active (with visible buffs) so it helps with that a bit. You can choose to focus on her defense or go with mixed bulk and focus on her resistance as well.
Tried to go a bit budget on this one since Vengeful and Special Fighter are a bit “premium”.
I’d probably just leave Bramimond the same, to be honest. Just make sure that you be some way of debuffing the foe (dagger unit, Aversa, Virion, etc) when using him.
I wouldn’t really recommend Ike either. He has a DC sword but he doesn’t have the resistance necessary to actually utilize it effectively without merges and a resistance boon, and maybe some dragonflowers. I’d recommend changing his weapon to something else, like Slaying Edge for instance, over using his Ragnell.
I can’t give much insight for AR, and Abyssals are where you’d want to have well built units or look up a f2p guide on how to perform them.