Silence the Sneak
You will need grenades for this mission, so head to Ammunation to get kitted out if you don't have any. Next, grab a big vehicle like a fire truck (the fire station is located to the north of the park, next to Uncle BJ's Deli). Take the truck to the sneak's hideout. When you get close, there will be a short cutscene showing a open window; this is where you need to throw the grenade.
Take the truck and park it against the garage door below the window. Get out and get about halfway between the building and the garage with the Blista (minivan) in it. This should be about the perfect place where you can chuck a grenade in the window with a full-force throw. It might take some experimenting, but it is possible; try using the sniper rifle to line up your throw if you need to. Watch out for any missed throws that might bounce back, and don't worry if you accidentally blow up the truck.
Once you get the grenade inside the window, the witness (along with a few cops) will try and come out of the garage door. The fire truck should slow them down, and all you have to do is roll a couple of grenades towards the truck, which will blow it (and the witness) up.