Please define "advanced"
I run a few shotgun classes...what I would consider the level II class, shotgun skills, is designed to work speed, accuracy, and manipulations to a high level. We do a little bit of slug work, a little bit of close quarters work, and a decent amount on using cover and concealment with the shotgun. It's a pretty high round count course, but it won't beat the hell out of you because we spend time working on recoil mitigation.
These aren't necessarily LE or civilian skills...they're just making you damn good with the shotgun and able to perform just about any task related to using one as a defensive weapon.
What I do in that class is pretty advanced and generally requires someone to have a good foundation on running a shotgun effectively to benefit. It is a class that builds true proficiency.
https://www.fpftraining.com/products/shotgun-skills
...but "advanced" is kind of an amorphous word that needs to be tied into some specifics. If you are looking for a class that talks about launched flashbangs, launching OC grenades, use of breaching rounds etc it's not the class you are looking for.