Marth is tricky, but remember, if ur going to do anything, remember spacing well, because Marths (like me) zone on said person. ftilt, utilt, and AA variations are good for on the ground for racking up damage, and in the air, use bairs, nairs, and surprise fairs (not against his airs, they are hard to beat, SHAD those). Use at least half-full charge AS, since he can block BAS with ftilt or A jab. grab quite a bit, use FP sometimes, and if he starts overwhelming u, use DT. Luckily, Fsmash is actually a good useful killer, since it outranges and lingers compared to his Fsmash. dair occasionally, but don't too often, or ftilt will take care of u. Careful of using shield, bcuz -> B and neut B are some of the game's best antidefense attacks. If he uses -> a lot, DT or even hit him with AS, since it's has a bit of lag pulling out of attacking. If he wails on u with fair, DT or airdodge, or utilt if u have to. Those are my suggestions, but yes, marth is a beast.