It is essentially the protagonist's story you're writing about and thus, he/she must have certain issues that the reader could relate to. Such as taking the bus or driving the car to work or take on the bad guys in a battlefield or sneak into the fortress. It's the choices the protagonist makes, rather than the best of his abilities (indeed maybe the lack of his abilities) which makes a charcter who/what we can love and cheer to.
Oh gods and freaks, I think I subconciously quoted JKRowling's Dumbledore of COS
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But still, the system works. The character grows, he loses his abilities and makes different choices. You question what if he still had the power and then which way would it force him/her to do the best thing or the right thing.
Indeed, I plan to make my character with super-potential and super-strong but her choices restricts her from acheiving top level. Not to mention getting crippled socially, physically, magically and emotionally (in that order). But remember, what makes a book is the characters. Plural. There are others that would support my protagonist along the way, those she had helped before. They wouldn't want her to give up no matter how much she just wants to lay down and disappear.
So it doesn't matter how tough/weak your protagonist is. Preferably, the tougher he is, the more people needed to bing him down. He could be a god with phenomenal cosmic power but it still comes down to either sacrifice his family/friends so their pure blood can heal the planet or rescue them to a temporary heaven and risk bleeding reality to the demons. It's the walk that goes with the talk
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