i just finished ShinRa Mansion and i have Beta on both my current ES Materia, got it on the first right after leaving the ranch, then as soon as i got the buggy i went back and got it on my second...
you do have to kill the Zolom in order to learn the Beta Enemy Skill you can not run from battle or you lose it, it does not cast the skill untill it's HP reaches 1000... putting the ES Materia holder in the back row and giving them Fire-Elemental in their armor slots helps a lot too also whenever the Zolom gets close to 1000HP make sure the one with the ESMat keeps their HP over 660 and stays in defensive position, when it cast Beta just cast it back and the fight is over... run for shore, heal thyself... it would definately be harder in an ESMat only challenge guess you're out of luck till much later... too bad cause Beta is like an instant "I Win" command early on...