If you are willing (and a little patient) to go farther in the game, you'll get a Fire Ring, which renders Beta a moot point when it comes to the Midgar Zolom. You're going to get at least 1 character knocked out, so make sure you put Elemental-Fire in the armor of the 2nd one (I used Barret), and it also helps to have Sense (so you can see the Zolom's HP). In addition, having a couple Limit's ready to go will also aid you in this task.

If you are trying to do this prior to going through Junon, good luck. His normal attack is at least 200-300 HP off, and Beta's going to hit even the Elemental-Fire combo for around 500-600 HP (or 900-1100 on a non-protected character. And YES, you MUST kill him. If you simply die and have a person "blown off" so you don't get a Game Over, the Beta will not be there!

Or you can just come back when you are like level 50 or so and wax the snot out of him. LOL