Here's how to use Deidara, with in - game tactics.
First off, you always want to use projectiles. Don't spam. Just use them. Mix it up with Chakra Birds and U1. They won't know what is next. I do warn you though. Stay away from Deidara's ground projectile combo. He finishes it off by charging them atop of a clay bird, doing a flip, then the bird homes in on the opponet. The reason to stay away from this is it WILL get you killed. Deidara does not accel in close range.
Second off, if you find yourself stuck in a projectile battle, charge. If your opponet is Deidara or Tenten or Temari don't stand there and dodge, DO SOMETHING! Chakra dash in and use Deidara's AIR combo. His ground combo is a no-no. The air combo will knock them to the ground giving you some time to do as you wish. Charge chakra. Prepare a next attack. Hell, even charge up a U2. Just make sure that you can avoid whatever they do.
Third, don't use awakening. It leaves you way more open to attacks. I use awakening most of the time, because I am that kind of guy. I sometimes lose because he becomes such a big target. This is the one time I am going to say it. Just once. If you are in awakening, SPAM. USE CHAKRA SURIKEN, USE REGULAR SHURIKEN. All of the projectiles get beefed up in this state, so SPAM THEM TO HELL.
I would like to elaborate more on this, but it is 11:30 pm and I am tired.
Look at Cinik's guide to deidara if you want less in - game and more technical terms and combos. I tried my hardest on this. And yes, I knew that sounded very dumb but whatever. Good night.