If your flipper rubbers are black or "super", changing them to red rubbers will make it much more challenging to control. Turning off extra balls is also a good option.
Increasing sensitivity of the pops and slings may help.
I'd go through the kickout mechs to make sure they are as strong as possible.
You could replace the black post sleeves on either side of the chair hole with thicker and bouncier yellow rubber post sleeves, and/or add a cliffy to make the shot harder.
Adjusting flipper alignment to a lower position could make traps and bear-ramp-backhands much harder.
Red spring in plunger would make skill shot much harder.
You could remove the outlane post on the left completely..
Or just swap in some lightning flippers!