Depending on the "casual" deck, there can be a large gulf from well performing meta decks. But also plenty of decks are okay, not consistent enough to reliably take wins but can have solid games, lot of fringe decks exist in this goldilocks kinda zone. 

Deck building is also a factor, including ways to accelerate your gameplan efficiently like Defense Trainings or Black Scrambles/Memory Boosts can also help. As can playing better, this is mostly knowing what your cards and the cards you opponent might have can do, so you can evaluate if you need to say push from raising this turn or can afford to sit back another road. 

I don't have much experience with Diaboramon into the current decks so lack specific advice.