If he's got a decent number of vassals, then engaging in unrestricted, pointless tyranny should help. imprison and then execute random, unimportant members of the court (and/or mayors). Preferentially target any that have decent skills, so you can whittle away his available council members. Make sure to include at least one family member. Execute his wife and marry him off to someone completely incompetent (unless she's completely incompetent already). If you can have him marry a young woman with serious genetic defects and execute any previously existing heirs, that's even better. Delightfully, all of this can be done during a pause except for the marrying - and even that can be done during a pause if you have an incompetent, genetically defective wife on hand in the court already.
If you want to be a cheap about it, leave any nobles that he might have that are landless (or can be made landless) with decent stats. The depth of their hatred for him should make them relatively easy to invite to your court when you head back to your own character.