To quote an amazing movie: "With a stick, while he slept."
You could also try wrestling him down to the ground, using punches, kicking him, or be cheap and get friends (be careful tho, NM backs Dekar well lol). But when he's playing Brawl and playing well, that man is unbeatable. So I would recommend trying to beat him in something else. Maybe a game of rock paper scissors ^_^ I wouldn't recommend trying to beat him with a gun or any high powered device. He can reflect them, stop them, or even turn them against you should he see it fit. They say that if you cut off his head you will gain his abilities. My triplet once tried this, now I am only a twin.
here's a link to Dekar that may provide some help
http://en.wikipedia.org/wiki/Dekar
I would recommend a small army or missiles at this point. Maybe get the entire smash community to help you out, maybe then you can win... but probably not.
Some more fun facts about Dekar that most don't know (they often use these to describe others, even tho they were originally said about him):
His shirts never wrinkle.
You can see his charisma from space.
The police often question him, just because they find him interesting.
If a monument was built in his honor, Mt. Rushmore would close, due to poor attendance.
If he were to train a monkey in Brawl, even that monkey would be able to beat M2K, but would never beat Dekar no matter how much he tried.
He does not need an alarm clock. Four bluebirds appear at his window every morning to wake him…no matter where he is staying.
You are more likely to win the lottery 12 times in a row, then to beat him in an all out match.
He gambled once. He was in Japan on a business trip and decided to put money down on Buster Douglas to KO his opponent in 10 rounds (great fight against Mike Tyson for those who never saw it. Great match. 42 to 1 odds.)