First off, make sure you're in an Extreme Rules match...
...now, set up a table (its easier to set the table up first, trust me on this) then build up your finisher. Store it, then build it up again. Now this time, walk over to the table, and press Y (on 360 version), the table will go alight, and your finisher will be gone. Now, build up your finisher AGAIN, then press Y when your opponent is groggy around the table area, then you'll slam them through it...
...hope i helped
Brawl FC: 1891-2896-6944
Rich's first ever Super Smash Bros Brawl Wi-Fi Opponent