If I have a let's say slayer 1 wizard 5 and I cast twilight knife and do sneak attack damage with it can I use studied target as an immediate action?
That's a tough one. My issue would be that the spell is making the attacks, instead of the Slayer. But that's very contentious. I would expect a lot of DM variation on this.
Personally, I'd allow it. It doesn't seem particularly overpowered.