This achievement is called "Yamcha's Premonition"
The description is "Win against Yamcha with the Saibamen using the Ultimate Attack, "Saibamen Bomb."
Whether this is a OHKO like Budokai 3 or not I don't know, but so long as he has 2 or less health you should be fine, hit Yamcha with a Saibaman ultimate, killing him and you should have your 15 gamerscore.