First things First , do not attempt the perfect boss runs until you have maxxed the Photon Launcher. Also if you get hit by the boss , simply die and continue and try again. The checkpoints are just before each boss in the game
Method for the Boss Fight
Start by backing away and shooting. As he gets close to you dash through him while keep firing. He will now fire 2 shots at you , keep firing at him while dashing through the 2 shots.
When he punches the ground be ready to fire up at an angle at him as quickly as possible. If you hit him twice he will drop back to the ground and the same fight continues. Once Tarus's health reaches 50% he will start using his laser. As he approaches you dash up twice to avoid taking any damage. You should be in the corner while doing this. When you drop down be sure to shoot him. He will now once again fire 2 shots which you must dodge through while you are shooting him. He will once again go in to the air. Be sure to shoot him down as quickly as possible and be ready to do another double dash upwards in the corner to avoid the laser. If he gets close to you in the corner use the opportunity to dash up and dash away from him.
Keep firing and using the method above until you get the quicktime event and end of the boss fight.