From a gameplay standpoint. In all reality, they could have taken him down by just hitting him.
That's an assumption that can't really be proven or disproven. (But I'm going to try anyway). Zelda is considered the wisest in the Zelda series. I feel there was a reason she stopped Link from blindly charging at Ganondorf after he found out Midna's death, but again I can't prove it like that. Maybe Zelda just wanted to have an extra precaution for all I know.
Also, "gameplay wise" doesn't really work here because when it comes down to it, you couldn't harm the G-man without those arrows. You could say "well in theory, the sword was all you needed" but if I say "well Ganondorf's horse was just so fast there was absolutely no way Epona could've caught up unless Ganondorf was stunned by light arrows." It's all theory. The basic proof that we have here is Link couldn't do it without those arrows.
I also haven't seen any good reasons to prove that Demise is weak. Even if Ganon is stronger, which is an arguable point, how is Demise not strong?
I don't know why you were looking for this. The debate was that Demise's actions didn't present an atmosphere of being more dangerous than Ganondorf, or being the ultimate threat. We used "only needing the Master Sword to defeat Demise, making him seem less of a threat than Ganondorf who required many items," as proof. We weren't arguing that he was weak but that he made himself out to be
weaker than Ganondorf. (Unless you count Ventus thinking he was "easy" which I wasn't associating with being weak as weak is characterization and easy is gameplay).