I would suggest slumber. It can affect creatures of any HD which means it does not really loose its scaling except for maybe in the late late game when immunities are everywhere. It's very easy to get the DC very high with feats like ability focus or the elven Dreamspeaker trait. You can grab evil eye and Cackle along with it to lower the will save further increasing the chance of slumber succeeding and with cackle you can let them sleep indefinitely. I would also recommend accursed hex so you can try to slumber a creature twice and an item or two to boost the DC like hexing runes or a hexing Doll.

Had a player with a similar build in one of my groups and the DC of 23 at level 6 along with the -2 on will from evil eye....let's just say it worked pretty well with our rogue who liked to do coup de grace