A Rosalina and Luma true combo that is easy to learn

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.

Pit and Dark Pit are extremely similar to one another. Here are a list of the differences from ssbwiki.

• Dark Pit's Silver Bow arrows deal 2% more damage than Pit's Palutena's Arrows at full charge.
• Dark Pit's Silver Bow also has slightly more knockback than Pit's Palutena's Arrow. However, It is slightly slower.
• Dark Pit's arrows have less range and cannot be curved as dynamically as Pit's.
• Dark Pit has no light rings to charge up his arrow.
• Electroshock Arm deals strong horizontal knockback which KO's sooner than Pit's Upperdash Arm if hitting foes off the side blast zones of the stage. It also does 0.5% more damage. However, it has less vertical KO power.
• Electroshock Arm also has the electric effect.
• Forward tilt deals less knockback.
• Dark Pit's Final Smash is different.
• Dark Pit's victory theme and poses (including quotes), and taunts are different. His "NO CONTEST" pose is also different.

Here is a video displaying how incredibly similar these characters are.

Also, according to these measurements, Dark Pit's perfect pivot is about 12.5% longer than Pit's perfect pivot.