Yeah, the first one is definitely killing faster for me.
Turnes out Anima Scutum ++ does work on Eel-K's small laser balls after all, but only while they're firing. If a laser hits the shield it'll vanish but if one gets to the end of the screen and you fly into it it does damage.
Still, that just made it a ton easier. You don't actually need the Arkus skill as Anima Scutum ++ blocks everything you'd be using them for anyway. I'd have this Achievement by now if it weren't for silly mistakes I keep making.