I'll try to help you. My Marth isn't very good, I'll be honest. I'll often disregard what I know during matches so I'll give you some tips for while you're playing.
1. Be unpredictable-
This is a wide category, just use mixups and don't develop a pattern for your moves. Pull out moves you don't often use to throw off your opponent.
2. Learn to recover well -
People at tournaments I go to compliment how I recover, apparently I'm good at it, and that just comes from practicing. Go into training, and just just off and practice recovering from different angles etc.
3. Punish safely -
This is pretty self explanatory, just use safe punishing moves and don't risk getting hurt when your opponent is the vulnerable one.
4. Don't play aggressively -
If you go charging in, you won't get far, trust me, this is my main problem. React to your opponents moves, and let them try to hit you, and weave in and out with your attacks.
5. Don't go for kills -
You may ask "WTF?" but seriously, if your opponent is at a high %, don't go rushing in trying to kill them, you'll just get damaged. Use setups for kills, and wait until the opportunity presents itself.