lol, you knock him down with a Dragonbone Smasher and wait for him to get back up. It's funny cuz the guy I soulsucked thought he died and sent me nerd rage messages. He probably didn't know he was still alive cuz he just stood there so I soulsucked him.
There's quite a few people who actually intends to get their souls sucked. The guy I got my +5 bladestone weapon from got me soulsucked for a trade.