I can definitely sympathize with your position. I live in the middle of nowhere, a part of the US that first got high speed internet a couple years ago. Needless to say, there aren't a ton of people around to play with.
With that said, me and a couple friends spent the better part of a year getting together on most evening and playing Brawl. Two of us got into the competitive edge a bit more, reading up on matchups and combos and such while the others were content to remain very casual. Pretty soon me and this one other guy pulled out ahead of the pack, and it became a situation where no one would play against either of us without the other also playing, in hopes that we would take each other out. We usually obliged, but even with only one stock left whichever of us won would often drop the remaining two for the win.
Now, this situation was fine for me. We both won a lot, and everyone would still play. However, the other more competitive person eventually began to lag behind me slightly as well, and just as I was beginning to notice a power difference when our group split up, all headed to different parts of the state. I still had a number of friends who were interested in playing, but at the release of SSB4 many had never even played before. Heck, if it weren't for the online play I'd rarely if ever have anyone to play with in 4 just because of the experience gap between me and my newer friends. Even when parts of the old gang get together, the capacity for online play and my continued passion for the game now have me leaps and bounds beyond my former sparring partners. So trust me when I say I can sympathize.
The best advice I can really give is a lot of what people have already been saying. Pick modes where things are more random and you have less practice. Play with characters who you don't play often or eve other than in the context of casually with your friend(s), possibly choosing bad matchups intentionally. Online 2v2 is a good way to play with your friend because it puts you on a team, not pitted against one another, meaning they don't have to beat you, just work with you. Also, if you have more people, FFA's can alleviate some of the stress as well, with temporary truces between other players to gang up on you helping level the playing field. Heck, if ganging up on you isn't enough of a gap closer, let them use custom characters while you stick to default, claiming your familiarity with the default moves and non-altered stats makes for easier use on your end.
I still saw one of the funnest things I've ever done on Sm4sh was with three very green and casual players in a local setting. All of them are friends of mine and have seen me play or tried playing 1v1 with me, and acknowledge that they do not have the practice to keep up. Instead, we put all 3 of them on a team with friendly fire off, went to FD, and they all went against my Little Mac. With each of them playing a different Mii fighter I lost, badly, because even with very little practice the brawler and swordfighter could collectively slow my advance while the gunner hurled projectiles in at me, a character without any range or reflector on a stage that I couldn't really avid his shots on. I always killed them more than they killed me, but at 3 stocks apiece I just couldn't keep up enough ground. I lost every time, they had a ton of fun, and I got to play with friends without them getting discouraged at losing so badly for the first time in years. I was the Bowser at the end of their Mario game, the guy with the gamepad in Nintendoland mini games, and it was hilarious.