Pretty much the difference between a mage that can only attack from a distance so their spells are telegraphed enough to block with the shield and someone who can remotely fire magic at point blank from any direction. With the former all you have to do is watch the mages while you beat down the front line.
I'm good with this but if Odette falls in love with Yuuki its a different ball game
It's more of, what's being presented to us - that Yuuki with Odette has actually more character development than Iris so yeah
Wait, all those guards, adventurers, and soldiers, but not a single one thought to try flanking the darn thing? It's not a deal breaker, but man, that's sloppy.
My guess would be the fact that there's too few magicians atm, so flanking is simply impossible. Remember that the physical unit couldn't get past the armor and brute force of the ogre, therefore the ogre could pretty much focus on the single magician that pose a threat to it.
That's probably the reason they're requesting help from the nobles and magic guild because, well, to get more mages to make flanking possible.