Is this a reference to that story about the strongest shield and spear? I'm pretty sure it is.

Indeed it is, although the emphasis of the comic is on Koakuma trolling the two lovebirds with her question.

Koakuma said:
So...
What would happen if the spear were to pierce the shield?

http://instantrimshot.com/

All this stupid impossible yuri preggo talk is making Remilia deathly ill. Can't say I blame her.

"What would happen if an unstoppable force hit an immovable object?"
"A child would be conceived."
GENIUS

ichbindereine said:
"What would happen if an unstoppable force hit an immovable object?"
"A child would be conceived."
GENIUS

Only in Gensokyo, Ladies and Gentlemen.

it occurs to me this is one of the only pictures by aoshima where a character (sakuya in this case) does not have their shoulders exposed.