One thing I would say is you're probably overdoing it with Seal skills. They don't stack, so having more than one of the same on the same team is a bit wasteful, and they're honestly not the most useful skills in the world anyway. Instead, I would probably try skills like breakers, Quick Riposte, and such, for the B slot.
Defiant skills kind of suck. There are much better options for A skills, such as Triangle Adept, Fury, or even just a bog standard <stat> +3 if you're on a budget. The reason they're not very useful is because you have to be under 50% HP for them to activate, and then you need to survive a hit at <50% HP in order to get value out of them, which very few units can do. Gwendolyn might stand a chance since she's an armour unit, but she might prefer something that would boost her Atk, which is fairly low otherwise. Tiki would prefer something that will increase her bulk further, but on a budget, your options there are a little limited. I gave her Triangle Adept so she can kill greens (and arguably does so better than Roy), but that depends on you having a spare Roy or Selena. Quick Riposte works well for her B skill, too.
Camilla is not going to get much use out of Darting Blow with that speed. Instead, she might prefer Death Blow, and just try to pack as much damage into those first two hits as possible. Though I would probably say that's still not her most ideal set up; personally, I would go with the Emerald Axe+ since her Atk will be pretty low without some form of Triangle Adept in the mix. But of course that would require spending 20k feathers on Arthur or Narcian just to take their skills, which you might prefer not to do.
If you're going Gronnraven+ on Cecilia, she definitely wants Triangle Adept for her A skill. This is because with it, she gains 40% triangle advantage over both blue and colourless units - two very common colours these days, with Reinhardt and brave Lyn and the likes floating around. Triangle Adept and raven tomes synergise perfectly with each other.
The others look decent enough. Klein is probably a better archer overall than Rebecca, though if you're in need of a mage tank, she should do okay for now.
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If anyone wanted to look at some of my own unit builds, I have all of my complete ones listed here:
https://fireemblemwiki.org/wiki/User:Moydow#.28Mostly.29_Completed_Heroes_units
There's too many for me to just list them here
I'm quite proud of the Felicia I just finished setting up, though:
She did have to eat an Alm, but luckily he was just a spare I happened to pick up while trying unsuccessfully to get Elincia.