What to do with Soren?

Id say keep him for sure, even if you already have a green mage arena assault is a thing and since reinhardt is often a thing its never bad to have green mages.
With that said, watersweep is great on any smokedagger low res ninja (ideally saizo) since they just need one hit regardless to do their job and debuffing 3 people can be better than just killing one. same thing could be said about low res mages with utility tomes (but there arent any yet, merric would be the closest thing)