They don't have to be online to delete them and one at a time is the best you can do. Heh, with 463 friends you're going to be at it a while.
The fastest way to get this done is to go to your profile, expand your friends list so you see the pages of avatars, right-click on one and pick the top option to open in new tab. (Some mouse wheels open the link in a new tab automatically.) Do this as many times as you want and then go to the page, delete the friend and close the tab. GL
p.s., 250 seems to be the magic number for friends. When you have over that many strange things happen on the list. You don't see friends online that you know are on line, the sorting gets messed up and the status is inconsistent.