Combos are ridiculously easy.

Go into training, keep sellecting Assist Trophies and Pokeballs. Use those for five minutes and you'l have a combo ranking of anywhere from 35-95 for that character. Use Kirby even to attack while let's say Gulpin has them in his mouth or the Staryu is blasting them.

EDIT: Double post, sorry. Geez. I hate my internet at times.

They are earned through challenges in the very same section in addition to the trophies available for that challenge (Party Ball & Stop Watch).

One is earned after 500 KOs, antoher after 30 hours of Vs Play time. There are five in all, if you have four that means there is only one left.

any tips for cruel KO's

 edge floating, basically jump off the platform, try to get the alloys to follow you, then hit up with a move to grabe the edge and repeat it again, do not jump oto the platform, keep at it until around 10 are dead or they get in a lucky hit. I did it with Ike.

For combos in training, just turn up the CPU's damage to 999, grab the cpu and keep hitting a to attack with a grab, each one count as a hit and you can easily rack up 50+ combos with little to no effort. 

I'd go 100 man mode with everyone after reviewing the numbers. Beating All-star on very hard will happen eventually if you've got enough coins... and should go faster.

does swallowing someone with kirby and walking off the edge count as a cruel?

Do you mean in the Cruel mode? Cause that's one of the Multi-Man Brawl options.

wait, so what about the ones like "Collect all stickers" that seems like it'd be a real pain. or collect all songs. something like that.