I don't remember if you HAVE to attack with a character in FFV or not. In many of the FF games you can skip a persons turn and go directly to the next characters turn. However, the character still has a full ATB gauge so if you never attack and only skip with one character, you can have your turn ready whenever Shinryuu attacks. If that character is a healer, you can heal immediately after an attack. If possible, I recommend trying that.