Is the plan to poke the guy with their feet until they get an 'ow'? That's where Tidus sees this going initially, though the rock Xehanort is still holding is giving him a cause for some concern - but also, completely understandable. He isn't about to tell him to put down his only source of apparently protection!

Tidus might've had doubts by the end of Xehanort's plan there, with sounding begrudging about the potential saving of another's life - but honestly, the whole stress of their current situation on a platform that's kind of floating around a chaotic landscape with two crazy vortexes in the background is troubling. And more worrisome.

"Okaaay," okaaaay!, "so, darkness - they gotta be conscious for that? What've you gotta do? Magic them?"

Does Tidus know anything about this darkness thing? No. Is this the time to care deeply? Also no.

"Whatever it is, try it!"

This can't go badly at all! (But also, sorry Xehanort, but spoiers: they're still people.)