Fanfic where dragons resist magic more and more as they age, and talk about Twilight's mustache spell

Question

I read a fanfic on FimFiction that I can't find.

Dragons resist magic, and that gets stronger as they age.

And because of that, Twilight uses Spike as a test subject for her spells because they can't harm him, and as a strength measurement. If they can affect him, then her spells are strong.

One example of such a spell is the one that gives him a temporary moustache / mustache. Not sure which spelling the story used.

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Quite sure it's not Entry 7, Part 1 (Boast Busted) - The Poisoned Barb's Tale - Fimfiction...

Or maybe it is? Somehow I read the first few chapters, which I totally forgot about? But is there any with a male Spike??

Answer 1

If you have misremembered and it was Sora who was the test subject then this would be Boast Busters:

"So, tell me again why I'm your magical test dummy, Twilight?" Sora asked as he stood in the Golden Oak Library, in the home area with Twilight Sparkle and Spike.

"Well, you have magical powers, so you must be resistant to some magical spells. Plus, Spike's needed to cross off the different spells I made."

Then:

"Ok. Now, hold still." Sora did as he was told, remaining standing where he had been since this morning.

Twilight's aura charged around her horn as she readied the spell she was going to cast. She then fired a small beam at Sora, aimed at his face, the glowing light disappearing after a few seconds. A second later, Sora's upper lip began growing a bushy mustache, similar in color to his own hair.

"Number twenty-five, hair growth spell!" Spike said as he crossed off the twenty-fifth spell.

Answer 2

I guess it must be Entry 7, Part 1 (Boast Busted) - The Poisoned Barb's Tale - Fimfiction...

Somehow I read the first few chapters, which I totally forgot about? But Fimfiction says nothing about the later chapters... However, downloading it would explain that... But is there any with a male Spike?? I haven't found any...

It even literally says "strength" in there.

"Because you’re a dragon!" Dusk groans out in reply, clearly not happy answering what I feel to be a fair and reasonable question. "Your scales resist magic! You’re the best candidate I have for strength based spell checks!”

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And I suppose the "stronger as they age" thing is both because it's a pretty common idea and comes from:

Story Shuffle: Left Shaken: Where Discord says:

a dragon as ancient

as Behemoth

is going to be largely immune to my more whimsical methods