This was how I died the first time, as I took it to mean that you had to to attack when the tail was up, to prevent it from countering. If they had said "If you attack when it's tails up, it will counter with it's lazer" rather than "Attack when it's tails up, it's gonna counter with it's lazer" I would have gotten the message, and like Skyblade says the default battle messages are slow, so it's easy to attack before the dialogue is finished.

Also, this was my first Final Fantasy, so I didn't know that there was a tradtion of not being allowed to attack the first boss at a certain moment since FFIV.