Poison is always a possibility to kill someone. You don’t have to be magically talented for that, you just need a reasonable knowledge of the nature around you. Even a childhood in the countryside would be enough to gain the necessary knowledge (at least I learned what plants to avoid because they are poisonous and potentially deadly).
The hard part about poison is usually to get your hands on it without anyone knowing, getting it inside your victim without anyone noticing, and getting away with it once the victim is dead. All three would be possible for Guen: access to various substances, having direct personal access and having a relatively solid position as the queen (which would mean that someone would have to procure extremely good evidence to go against you).
It wouldn’t be reasonable to have her poison Scarface though. While it would be theoretically possible that she could brew it the night before, it’s relatively unlikely. She is the Queen riding on the battlefield with two personal knights as a guard. If she isn’t soldier Guen fighting on the frontlines (which, by the way, seemed highly unusual to me), she wouldn’t expect to see any action. And even if she does, it’s not the kind of action where she would need some poison. At least as long as she doesn’t start fighting with poisoned weapons, which would be an extremely bad idea on a battlefield (the risk of wounding yourself or one of your allies would be way to high).
I don’t think anything other than good old violence would fit for killing Scarface. Even the magical ritual I mentioned before wouldn’t be plausible since it isn’t likely to be set up in the middle of a battlefield with only a couple of minutes to spare before Arthur shows up.


He’s the kind of charming, joking but considerate character I’m particularly fond of. Which makes choosing between him and Arthur that much harder >.<