Are there any taunts that can be used as an attack

There is still secret taunts like the Fox one. There is one when you are Pit and the stage is Palutena's Temple that Palutena gives you advice on the fighter you are currently fighting. It is called "Palutena's Advice".

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.