The Midgar Zolom has 4,000 HP. Once you deplete its HP by 1,000 it will raise itself up into a standing position. Reduce its HP by a further 1,500 and it will counter with Beta. Beta can only be used once per battle as it is triggered by the Zolom's HP passing below 1,500. Beware though, because when its HP is lowered below 2,000 it may knock the character that attacked out of the battle.