Number one is pretty simple. One of the methods (and my personal favorite) is to pick Yoshi, quickly roll back to get the bat, do a small jump and use Yoshi's down aerial attack. As soon as you land, jump back up and repeat. This move hits a bunch of times and racks up damage fast, so as long as you're accurate and quick, getting 400m will be easy.
The normal 15 minute melee approach is to choose DK and stand under one of the platforms on the left or right and just keep pressing Down+B. If one of the wireframes is somehow hitting you from above, use can probably quickly wipe them out with the up tilt. Another possible method is to choose Mewtwo, stand on one of the edges of the stage, charge his neutral B move and just hold it. I haven't done this method myself personally but I heard it was supposed to be effective. Also remember that both of these methods require a bit of luck to last the entire 15 minutes, and the DK method is pretty labor intensive.
[ChanseyMaster]