Considering none of them have been affected by the Cwyld (and I could be wrong but it seemed like this creature isn't well known or understood in general?), Waver would definitely want to research it in the name of science! He would advocate for figuring out what makes it resistant to the Cwyld (or if it's not resistant, if something else is at play) and also try to learn more about its origins, what kind of creature it is, any magic it has or magical properties, etc. For the researching part, he'd want to consult with residents of Dorchacht.

For the actual capture and containment part, Waver will suggest an enclosure surrounded by running water, ideally powered by spellwork that would allow for the water to be present in an actual cage-like structure rather than only on the ground, just in case. Something like a waterfall on all sides? It has to be running water, so maybe enchanted walls with water coursing inside them, either as a separate moving layer on the inside or outside of the enclosure, or two sets of walls with a layer of water that can move between them. For this, they'd need people to engineer and help build a complicated enclosure and Witches to enchant it and make sure the spellwork is refreshed when needed.

Somewhere out in a cleared part of the Wild near the outpost or the city would be best so researchers can go out and observe it without endangering people who aren't specifically there for that reason (he'd definitely be against trying to bring it into the city proper).