No mindgames in this matchup: turnip-based rushdown is the key.
you ALWAYS need mindgames for everymatchup, especially counters. The only time you don't need a mindgame strategy is when you're facing a n00b
Also, while turnips seem to work, they're not exactly reliable. Marth's sword gives him a range of not being hit by several projectiles (turnips included). So, if you throw a stich to him from a certain distance, not only will he stop it from a ftilt, but if he was to short hop, fair the stich, and L cancel, he could grab the turnip and throw it back at you.
A Marth is gonna be agressive with you because he's confident about his tipper. Remember, space yourself well and attack. don't FC too much unless you know it's gonna hit.
in reference of DI, when you're at low percentage, you gotta DI away from him. but, when you're like around 80%, you gotta DI to him (or it's the other way around. Not too sure). If you DI wrong, you'll face his fsmash or spike badly.
Again, playing smart and good spacing is key. You can't just be agressive with him or defensive. You gotta use all those tactics in a mix order. Like, attack, defend, evade. or evade, attack, defend. or even attack, attack, evad. there's many things you gotta do.
And once he's off the stage, you gotta make sure he stays out. A Marth could grab the edge and short hop up to do a nair, which is effective. Or they could just Up -B, hoping it'll hit, and then just jump up just as swiftly. I think the best way to take him out is to dsmash just before he's close to the edge when he Up - B
Now, if you get fling off the edge, there's gonna be some problems getting back on the stage. Marth could sweet spot you good because of his tipper. Even when you grab the edge, if you either roll or just get up, he'll wavedash back and fsmash you FTW. This is likewise if you're hovering above you when he short hops/jumps and either Uair or Dair you (mostly Uair) FTW. Your best chance of getting back is to fake him. If you're high over the level, float over to the stage. If he tries to attack you, Up B yourself and try to land close enough to the ground and away from him as fast as you can. If you're Up-Bing yourself to the field from the start, then just land on the stage and when he attacks you, move back and just fast fall yourself down. DON'T try to grab the edge unless you're sure you could fake him out (such as jump up, air dodge as soon as he fsmashes)
The best you can do is stay close and watch out for what he does. Good luck, you'll need it.