Building Soren

Well I'd normally say a raven adept but at a bane that defense stat will be pretty damn awful, to the point that brace Bow Lyn/Cordelia would be able to kill him despite triangle disadvantage. Most people are however making use of Mukagirninstead which should logically keep him safe enough to bait kill Lyn with that 40% damage boost on his first counter attack.

He's not the ideal raven mage in green (low speed and high Attack and defenses is generally preferred) but if you really wanna keep using him on a team it's likely the best way to use him differently from a properly made Nino.

In this case you do have the advantage of being faster than most other raven mages even if physically frail (avoid brave lance Effie attacks). You don't really need a blue breaker skill for anyone but Effie if you expect to see her often, but if not then G tome breaker to kill enemy Nino/Julia/Spring Camilla might not be a bad idea.

I am in the same situation as you. Mine is currently 5*. I plan to give him a raven tome for the purpose of blue/colorless nuking and an anti-meta unit that can check Reinhardt and Blyn. Mine currently has TA3 and Atk Ploy (I fed him summer leo). Also +atk -def with Summoner support. He does really well in my trials now. Hopefully he's one of the bonus units too in the next trials since it's PoR related. Good luck in your building!