For me It was pretty hard at first…
But then I found out how to play!
All you have to do is:
1) Attack the sandbag as much as possible, increasing it’s hit percentage. (projectiles are good for this, like Fox’s gun or Link’s gale boomerang, arrows, and bombs)
2) Stop attacking when you have about 3 seconds left.
3) Grab the Home-Run bat.
4) Do a smash attack with the bat and send the sandbag flying. (to do the Home-Run bat smash attack with the Wiimote & Nunchuck controllers hold A and right.)
That’s It!
Here’s a Video:
Super Smash Bros. Brawl 