Well what's the trouble you're having? Don't change characters unless you absolutely HAVE to--Let's look at your options. Who's your main? How can they turn Luigi on his head? (insert a wiffed Luigi up b here)
Luigi is a hard matchup for many characters, because of his incredibly good grab game, his overall power on a lot of his kill moves, and (in the right hands), his incredible recovery options.
A big thing for Luigi, however, is relying on grabs, in quite the predictable fashion--Fireball into grab is incredibly common. Instead of shielding or getting hit, jab the fireball and then jab him as he comes running at you. If he rolls backwards, which is common among FG players who are running low on options, find a way to punish it and keep the pressure on.
Most Luigi players don't have the mashing skill of Boss. By saying this, I mean that (most of the time,) Luigi is incredibly predictable while recovering, thus making him easy to gimp. Once his double jump is used, he's essentially a sitting duck. Take advantage of this opportunity. Good Luigi players will mix up side b and down b before exhausting their double jump, because they know that without his double jump, Luigi's recovery is as bad as Doc's. (Once again, we're assuming that this player does not have turbo fingers, in which case, don't be afraid to punish him on the lagtime of his down b--He can only use it ONCE to gain momentum when airborne. After that, he must refresh it by landing.)
Luigi is an incredibly good (and devastating) character when placed in the right hands. However, keep in mind his flaws when you battle him and take advantage of him. Don't let him control the match with his grab game, and if you get him offstage, don't be afraid to chase and punish. His air game is good, but he has very predictable patterns with a lot of lag time. Be brave. Hope this helps.
Edit: DI his downthrow INTO HIM at early percents. This limits his chop into jab or chain grab options, usually eating only one chop, or a uair/bair. At high percents, I prefer to DI away in hopes that I can get out a double jump while he begins to whirl. I've trained against a good Luigi on FG and the only way I could truly avoid downthrow into cyclone was to NOT GET GRABBED. It was faster than both my nair and even my up+b... And that's what? Frame 3?