Biggest tip for you straight off the bat: Never settle for a starter/structure deck in DN competitive. Remember that you have aces to every card in existence, so build your deck accordingly.
Use a raigeki, if it help your deck.
Use twin twisters instead of MST or whatever else you've been using for hitting backrow if your deck fits well with it.
Don't hesitate to use anything expensive!
Solemn strike is a great way to boost many decks!
A copy of Solemn Warning can save your butt big time!
If you can write out your deck list for me, I can try to give you some advice.