"If we can't stop 'em from attacking, we're gonna have to put them down." Which given they weren't the only fighters, some of the others were doing already. And unfortunately, Tidus knew the area as best as Venus, surveying their surroundings quickly without much hope.

"What do you wanna do? What if-" ugh, this was probably not going to work, but- "What if I tried putting the sleep spell into your rope? I don't know how else to cast it!"

As easier as it might be to just do it directly onto the dogs--but it's not the way he learned it, and there's no time for trying other means.