I don't play Legion, but I'll offer a few observations. I'm going off of my memory here, so if something doesn't seem quite right, well, it probably isn't.
The Nephilim and Angelius are pretty good with Rhyas's feat. Both are able to get over or around intervening models, and the feat helps you get to the key models that need to be assassinated (Unit leaders, support solos, etc.). The Carnifex is perfect for her Rapport spell, but that's not to say an Angelius, Seraph, or Teraph wouldn't appreciate it either. Between Dash and her Feat, I'd say melee infantry would be best with Rhyas, perhaps Blighted Ogrun?
Saeryn likes to have one big warbeast around for Respawn. With Blightbringer, you'll want a big warbeast to cover more area, but you'll also want it to be fast to move into position. The Angelius can charge a unit, kill a guy, then move 3"(
IIRC) to cover the most enemy models. Then cast Blightbringer and *poof* dead infantry. Then, if there are any survivors, you can use Saeryn's feat to protect it. For infantry, again it seems like melee infantry is the better fit. You could just run everyone into melee and use her feat to keep them safe. Next turn, chop 'em up.