I heard that charming wisps to kill other wisps is a very effective way to farm GLSs. Only like 2000 kills or so, and you can just buy the ring!
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Seriously though, it looks like you'd be able to work on the ring camp with your mage. Might be hard to get it set up though, and it'll be slow to do. Usually people round up all the possible PHs and kill them so that they get respawns at that double spawn point mentioned on the wiki and they keep killing both those PH respawns every 30 seconds or whatever. At only 20 you'd have to put some effort into killing the level 15ish PHs, so you'd be getting a LOT fewer chances over the course of an hour for a spawn than a 40+ character would.
For what it's worth, I say screw the ring. Breaking charms with invis is bothersome, but it barely diminishes the leveling power of a chanter compared to one who uses the ring to break charm, especially if you make use of a cheap clicky (like a telescope or kromzek surveying scope) to reset for a quick nuke or root after the invis and/or level in more spacious areas (not even outdoors necessarily, just with enough space to get a root off before your pet runs back to you), etc.