I'll say this of a death battle in Errant: It's a good job Sarin Aoki is the most upstanding moral of everyone in the comic, because if she wanted you dead, you'd be decapitated before you could move. Sarin isn't necessarily the most powerful person in the comic (Her primary weakness is her inability to channel much power into her body; she's a glass cannon), but she's the one who'd be the most efficient at killing humans due to two characteristics:
1. She's good at sensing magic so anyone magical can't hide from or sneak up on her.
2. Her magical abilities are mostly based in long-range energy projection and her aura slices things (you can see her aura in the prologue leaving cuts all over Ambrosius. That was her at fourteen using it instinctively. Wait til you see what it can do now we're past the time skip...
)
Ironically, a person with no magic and a sniper-rifle would be the most effective person to defeat her. A magical knight with a sword though? Oh, they're super-dead. Unless the fight started with them standing literally like a foot apart.