My best advice is to just play 1vs1 and free4all with a character you don't quite understand yet against lvl 7/8 computers for a while until you can play with every character good enough to use their fastest/safest moves. For most of the time, camping at the edge and planking off a few is a great way to get some safe kills. Here are some ideas for each character in this idea.
Limited to basically uair the ledge (safe) and plank
Mario/Luigi/Diddy/Peach/DK/Kirby/Ganon/Captain/Lucas/Wario?/Bowser?/Shiek/Squirle/Wolf
Limited to other air moves to the ledge (safe) and plank:
Samus: Uair, Fair, missle, charge shot, nair
Ivy: Fair (space far away)
Lucario: Fair (space far away), Uair, Nair(sorta..)
Dedede: Fair, Uair
Ness: Uair, fair
Olimar: Fair, Uair, throw pikmin (purple <3)
Tink: Nair, fair, uair through the stage a lil.
Jiggly: Uair, pound, fair, nair.
Ike: Jumpping fair off the ledge.
Rob: Gyro, fair (sorta), uair, bair <3 (gyro cancel so you can face away from the ledge)
G & W: Fair, Uair, Nair.
Metaknight: Durp
Pit: uair, fair, arrows.
These are just ideas that work to abuse their stupid AI. Jigs and metaknight and lure them to jump out and try to get you and fly to the other side. Pit might be able to do the same.
Some characters can just spam and survive:
Snake, Marth, Wolf, Falco (f-smash, laser and forward-b to keep it fresh), Bowser (upb), etc.
Really it's not that hard to do regularly if you play well enough without these gimicks.
EDIT:
So I ran into 100 man melee just to test some of my stuff and I figured out that the fan is your best friend. They're really common and you can simply stand on top of a platform (or any where really) and keep throwing it into the air over and over again and it will float slowly to the ground. Anything touching it will die. On average I can get at least 2 or 3 per try and it makes things really really easy.