Radiation is the best because you can still build for viral and get the bonus to alloy plus the crowd control.
Actually a damage and multishot combo like that for the Karak wraith (or especially the kuva version) is great on it's own, but you also have heavy damage to grineer, so a viral+slash build would be pretty brutal with that.
You may want to actually hold onto it for yourself imo, unless you already have something better.
You are on a wiki which has this information, just go to any kuva weapon page and it will say lol.
Atlas is immune to them while on the ground, i think Titania's spellbind helps ignore it, etc. Personally i don't have time to put an exilus forma on every frame i own to use primed sure footed so i just use handspring to get out of any stun (even the worst one) in just a second.
You sure do ask a lot of questions lol.
(note: i don't know how updated this website is or how relevant it is for you) https://riven.market/list/PC
He can use Smite Infusion to give everyone bonus radiation damage which has a small bonus against the Eidolon's health (and to it's armor), as well as his Hallowed Ground providing immunity to knockdowns and magnetic procs during the downed periods.