Can try poor mans impact driver. Stubby Phillips gripped in you hand holding it against bolt. Apply pressure in direction to loosen it and while you have that pressure applied hit screwdriver with a hammer. Have someone hold your bike and watch your hand.
If you striped out the Phillips head on the bolt then might have to get an "Easy out bit kit"
"Rigid screw extractor set" both the same thing just depends on where you go to buy it.
Got my set from Harbor Freight for $10 I believe. Had a fairing bolt I had to get off. Gotta love dealerships who follow torque specs! You will also need a drill and steady hand. Make sure to use the correct bit when drilling. Bit need to be smaller then the threaded part of the bolt.
Once get that off might need to cut some of the plastic on the throttle to be able to slide on the new grip and have it look right. You'll understand when you see it.
Good Luck.