Natsu's already HAD those moves, he's just never used them. It's not like we get a spreadsheet of everyone's abilities and movelist at the start of the season. But like, What'shisface said in one of the earlier chapters, Natsu's flame is connected to his emotional state - the more angry, frustrated, or focused he gets, the more powerful his flame magic becomes. That and Luxus surely had to use up a ton of magical energy to activate Fairy Law like he did.
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"Focus entirely on me, you ordinary soldier."
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