Yeah, what poopcannon said is perfectly correct. I was stuck on this for a while too because the walkthrough I was using told me to hit right, left, right at the start of the battle with Sabin...

It wasn't until some tries later that I screwed the walkthrough and finally realized that I have to bring his HP down and then it tells me the instructions later in battle (being really left, right left). It shouldn't be too hard if you follow poopcannon's directions.