You don't need to keep the meter full. You just need to keep it from emptying out. It's designed to seem harder than it really is—the bar fills much more when you push any button while it's <30% full, whereas inputs above that threshold will barely budge the meter. When people fail this mechanic, from what I've seen, it's typically because they spent a lot of time mashing their heart out trying to refill the meter, only to tire out their fingers before it's over. You really don't have to do that.

That said, the fact that the entire party wipes if even one person fails the QTE is annoying and, frankly, a little nonsensical. Just let the people who failed it die, and let them be raised after the fact.