Try to take it easy and slow in the beginning, try to predict his movemens, follow his movements and what he does, your opponent starts becoming more and more predictable which means an easier K.O. Try to use characters that are well rounded for though situations.Hope it helped.
Don't button smash, don't run off edges, don't be characters you suck with, don't throw smartbombs then run into them, try not to be predictable (Spamming attacks, using a pattern).
always try to make you're oppenent(s) damage % high, about 100-150 should be good for most characters. thats when you try to use smashes. don't spam them though, they begin losing power after a few uses. if its a heavy character (bowser, ganon, dedede), you may want to bring thier damage % higher then 150 to effectivly knock them off the map. just dedede is a pain 'cause of the floating thing. if you see him floating back, try to hit him when he gets close to the edge unless he does the stomp move. just clear out and wait for him to land if he does, then just smash again.
Dodge-roll is a pretty useful technique, a well balanced character also helps. I find it easy to use Mario, Ness, Lucario, and Toon Link. If you can deal with the lack of speed Ganondorf can be a good choice. If you want a quick victory you could use Ike, he is a bit slow but if you land a smash attack few characters can survive (yes, I believe Ike is incredibly overpowered, which for the most part is very true). Pit also plays very well. As far as characters go it really is just a matter of preference, I mean you can win with just about anyone once you get used to their advantages/disadvantages.
I love how the internet is its own fantasy adventure novel. Person A seeks guidance and the Trolls devour him.