As long as you can keep your marker inside of the bar, you're fine.
(very minor spoilers) . . . . The hardest part is basically right when giant Scales grabs Fox. At that point, be ready to fight a MASSIVE twitch of the bar. . . . . (end spoilers) --- A staple formula for FF villains: "They only do this, but lets kill them anyway!!!"
Try to focus ONLY on the bar and ignore the action on-screen. (Not as easy as it sounds, I know) and just keep the line near the middle as much as possible and as mentioned above, watch for the major spike. --- I set the laser printer on stun