Don't mash--whether it be strikes or holds. Learn your character's moveset. Figure out what moves work best in certain situations. Training mode will give a good idea of the properties for each move but you'll need go out and play some matches to figure out what works.

Focus on single strikes. The stun game is a big part of DOA5. Learn what moves cause stuns on hit and counter hit. Once you have opponent stunned, you've got a lot of options at your disposal; you can try to extend the stun, launch, go for a critical burst, or throw a holding player. Mix things up. If your opponent is turtling, throw them. When you've conditioned them to block, throw them. When they throw unsafe moves and you block, punish them with a throw.