This is easiest done against AI opposition as human players tend to be cautious and avoid the edge of the pitch (though it can also be boosted)
I personally find the best way to crowd surf someone is to use a high strength piece with Frenzy (allows you to get 2 pushes on a player, giving you more options)
I used a Lizardman Saurus with Block / Frenzy
NOTE:
You can also push someone off the pitch even if you don't have them immediately between you and the sideline.
Example
0 = Empty Square
E = Elf
S = Saurus
A = Attacking piece
000|
0EE | <- Sideline
000|
You can crowdsurf the elf to the right if you position an assisting piece immediately behind him (as below)
000|
0EE|
00S|
And push him from the top left square:
A00|
0EE|
00S|
With a push result here, you will push the defending piece off the pitch.
Good luck