Here is an actual technique that works:
To understand the best method of capturing any Pokémon legendary or not you need to understand capture mechanics of the game. Gamepress did an awesome article explaining in detail How Pokemon Go Capture Mechanics Works. Curve balls, circle size and Razzberry/Golden Razzberry all affect the catch rate. One of the biggest modifiers is how the circle size affects the capture rate. If you are able to land the ball in a smaller circle it will improve your capture chances.
If you press on then ball the capture circle shrinks and resets. A fun Easter egg; press down on the ball until the capture circle is at the smallest size that you are confident you can reliably hit. Carefully release the ball and the circle will remain the same size until you touch the ball again. During an attack the circle is locked until he finishes the attack so you will be able to spin the ball for a curveball and can release the ball at the right moment without having to worry about circle size. After the Pokémon attacks, you have a short window to throw the ball that will guarantee the Pokémon will be stationary and won't dodge or deflect the attack. Since the circle was set at a size before the attack a well thrown ball will always give him a better catch modifier and improve your chances,
Patience is ultimately the key. It may take more than a minute for some of these beasts to attack, but if you wait for the right moment you can reliable hit great throw curveballs or better a very high percent of the time. It may take practice but you can make great throws and vastly improve your chances .
The final elements that is in your control is working with teammates to have control of many gyms to get the team bonus and to power up pokemon to improve your damage output and to get extra balls through the damage bonus.
Try these proven techniques and you will improve your odds at catching the legendary beasts.
Good luck trainers!