Get good at the basics and then you'll do better in FFAs. Though we're a competitive forum that focuses purely on 1vs1 and Teams [with damage on], our guides and videos will make you a better player. By simply becoming better at 1vs1 you're going to be better in FFA, regardless of how many people gang up on you. If your pursuit is to become genuinely good at the game, find the character you like and go to their character section.
General tips for FFAs ->
Learn to space. Attack so that you can harm your opponent, but they can't hit you.
Don't dive into situations that are risky.
Pick an area on the map and stay around it. Don't let your opponent come into your space.
Edge Guard your opponents. If they are off the stage and you are near, without anyone threatening you, prevent them from making it back.
Shielding is incredibly important. Perfect shielding even more so.
You're invulnerable for a dumb amount of time by just spamming Spot Dodge.
Learn to DI. We have tons of articles on it. You can live for SOOOOO much longer when you know how to do it.
Avoid spamming your kill moves. Brawl depreciates your attacks in power and knockback the more they are used.
'Snipe' people that are above 120%. Don't let people live to high percents.