He's always looking at you, as such, it's simply easier for him to lock onto you as you approach his doors, so the closer you are, the more likely you'll be shot. Move diagonally, using the rocks as cover. Whenever you're shot, your digimon's health will be halved; go behind a rock and administer a healing item. Don't let your digimon's stamina be reduced to 1, since you'll have to restart. When you can see the three doors, approach the one marked with an O.
what i meant was when you are at the screen, go up and turn left, then up again and turn right, up again and turn right, up and you will be at the right door. even if its an X, just go towards it and you will start over again until the right door is an O.