I am having the worst luck with skill farming at the moment, and I am astounded at just how terrible it has been. I have used Sartorius against any legendary duelists in GX, multiple farming attempts (about 6-7 per day at minimum) at around 7900 points for the Lv 40 event and Lv 60 prophetic, and about 18-20 legendary duelists at the gate, and I have yet to get any of his skills other than through level up... and he's Lv 31 now for reference.
Basic gist of my irritation:
When you really wanna play a favorite archetype of yours growing up and the game has the ultimate trap card to prevent it: The RNG System
Ironically I have gotten a lot of skills for Aster through this event. Like he is only missing Draw Sense: Fire and Light. Compared to just doing ranked this event has given me a lot of skills and I'm not even farming Sartorius for 8 drops at the gate.
They did somewhat recently Nerf the chance for skill drops against NPCs meaning you have a better chance at farming if you play PVP duels. If you don't fancy getting your 🔥 handed to you in ranked you can always just play casuals duels to farm.