Flash.
Even without all the major speed tricks, what's to stop him from simply vibrating and ripping out a major organ, like his heart or brain?
In the race Mxy set up, a much slower Wally did vibrate out of Supermans grip, which proves his durability won't stop a vibration attempt.
Barring being able to actually vibrate into his body, he should at least be able to vibrate through any punches and could pull what Walter West did to another character and mess with the electrical impulses of his brain and other organs.