What character / attack / stage combination can get me a combo of 100?

There are two easy ways to get a combo of 100. In either case, you should choose a stage that won't interrupt you. I recommend Final Destination or any Omega stage for the sake of simplicity.

• Create 3 smart bombs next to your opponent. Move away, create a fourth, and throw it at your opponent. This should cause all 4 smart bombs to explode, each dealing 25 hits to your opponent, totaling 100 in just a few seconds. This is fast and works for any character, so you only have to do this on 4 characters to get the 400 combo total across all characters.

• Select a character who can hit rapidly while holding an opponent, such as Ness or Fox. Set your opponent's damage to 999%, grab them, and mash the attack button repeatedly until they break free. This should get you a combo of about 120 hits.

As for what exactly constitutes a combo, it's the number of hits against your opponent before they are allowed to escape the hit-stun. This can be multiple hits per attack, or even 0 hits for attacks with no hit-stun, like Fox's laser. You can read more information about it here.

There is a way to get a combo of 999 in the 3DS version, although it's a little bit cheating.

Use a default Lucario, pick Fox as your opponent, and enter the Corenia stage.

Get Fox to the part at the back of the the top wing with the curved edge. Be sure to make him as close as possible to the wing. Bring everyone to 42%, the use Lucario's natural special while facing away from fox. Make sure to keep the damage around 42% or he will escape. DO NOT press B again or he will escape.

If you are using the Wii U version, make a custom stage with a curved side like Corenia.