Wake her to try the flash trigger.

"Alright, now, wake up," Natsu said.

Lucy blinked, and shook her head. "Well, glad that's over..." she said, before catching sight of Natsu's grin. "It's not over, is it?" she asked.

"Nope," he replied.

"...why did you bother letting me get dressed if you're just going to have me naked again?" Lucy asked out of genuine curiosity.

"Good question," Natsu admitted. "But I also have a couple good answers. For instance, it's enjoyable just watching you strip. Also... Flash-Unaware," he said.

Without even realizing she was doing it, Lucy reached up and undid her top. "Also what?" she blinked, unable to perceive the trigger, pulling open her blouse and giving him a good look at her tits.

He smirked. "Some triggers just work better when you're wearing clothes."

She shrugged as she obliviously closed her blouse. "Fine, but can you please not make me do anything embarrassing?"

"Not gonna happen," he replied, making her pout. "Now... Flash-Aware."

"Eeek!?" she yelped as her hands reached to undo her blouse again. "Now what!?" Her top opened up, and put her boobs in clear view for Natsu. Lucy went bright red, but didn't move until five seconds were up, to which she hastily closed her top. "...I wish I could control myself," she huffed, questioning why she still couldn't feel mad at Natsu.

"Ah come on, this is fun!" Natsu chuckled.

"For you maybe!" she grumbled.