Go to your friends list and 'View All'
On the friends you wish to delete, right click on their icon and 'open in new tab'. This will load their profile in a new tab
Only open 5 - 6 tabs at a time, go to those tabs and from there as WGTIcon said, 'remove friend'.
Close those tabs, yo are back at your friends list, open another few.
You may be able to open more than 5 - 6 tabs without flash crashing and the 'remove friends' button no longer appears.
I can usually open about 9 - 10 before i dont see the 'remove friends' button on their profile any more.