It's pretty obvious implication
He has the attack power to damage Madmole. By precedence, he has high attack power, taking down Ajeel and Sayla.
IDK why this is even a question lol, the dude's entire shtick is brute stats and no magic lol. He has always had high attack power
It proves that he can just stay put and Lucy will just exhaust herself. Lucy has nothing which can take him down
Should be pretty obvious I think

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LFD Natsu is so strong that Aquarius is afraid he'll accidentally end up killing Lucy when he's just trying to put her down. Sure, this is LFD, but LFD isn't that much stronger than base Natsu, it's at most a tier above base.