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

Going by the example set by previous games, training mode defines a "combo" as "series of attacks that keep the opponent in hitstun", possibly with one frame of escape window (not sure as of this time). So, you need to get in 100 hits without allowing the opponent a chance to act.

Being that it's hard enough to get a legitimate combo of 20, you're going to have to cheat a little bit. Spawn a bunch of Smart Bombs and explode 'em. Depending on a few migitating factors, the resulting combo will be somewhere between 50 and 100. Give it a few tries and you'll get it eventually.

As for the "400 across everyone" challenge, that's an average of about 8 per character. The chunk of 100 will drive this down to about 6 per character, which is easy enough with a single Smart Bomb or even characters' infinite jabs.

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.