So is Youki going around collecting helpers, to help until Youmu is back up and about.

Youki can't lose. Look at panel 2. His face screams Undefeated of the East.

F.I.A wrote
The touhouwiki worded it as "slash" instead of "flash", so I will need confirmation on this.

It's just a minor detail. 「閃」 literally means "flash", so if you take it that way, it means that Youmu covers a distance of 200 Yojana very quickly (and follows up by executing a slashing attack). If you use touhou wiki's wording, it just means the rapid movement is an integral part of the slash attack.

I wonder which who will win this. Youki and Youmu both can move about 27000 times the speed of sound (from Youmu's calculations earlier) but Sakuya can stop time.

So am I to understand that Youki's dialogue in the last panel is meant to be "old-style" speaking? Like the Japanese equivalent of Elizabethan English.

Ipswich67 said:
So am I to understand that Youki's dialogue in the last panel is meant to be "old-style" speaking? Like the Japanese equivalent of Elizabethan English.

That or it's meant to sound badass. Or both.
Admit it, "cleft in twain" sounds a lot more badass than "cut in half."

It just says in normal Japanese, "You'll be cut in half." Liberal TL done for badassery.