
It's the face of someone who doesn't know how to lie, is in pain, and tries to make a "reassuring smile" to say "everything will be fine".

I truly believe that with all my heart ,unfortunately it doesn't make me feel any better. it seems to be feeling of losing ,his back may really be against the wall this time, if that's the best that he can give his crew. I miss the luffy that was so sure it doesn't fit his personality at all. but if we are to truly invest in the story we have to believe that sometimes even the bravest strongest Heroes don't know what to say and that's all they can give ┗( T﹏T )┛
I'm glad law didn't see it though I truly think it would have ripped them apart like he hasn't had enough trauma ╥﹏╥

Luffy is Human, which means he isn't perfect and like any other person, he can also feel fear. If he was so sure and confident as when he begun this travel, that would be dangerous for him and the crew. When your life is in danger and you have rivals and enemies, some of whom are bad people, then you have to be more mature and cautious in order to be a good Captain. If Luffy wasn't a bit afraid in this situation, he would be a fool. Plus, a man can only be brave when he's afraid.

Anyways, the Cora-san scary smile is a bit creepy, but I believe Oda wanted that type of smile to be "funny"... So maybe we're not supposed to take it too seriously

Be that as it may it's not funny it's heartbreaking, all the same for the record I agree.
With that being said:
IMO,the only reason he's not as confident in this battle is because his opponent has no personal stake in luffy's life or crew. Meaning he's not like "doffy" (more like he's less than Bellamy) this guy hasn't done anything to him personally (or his crew that luffy has knowledge of) this guy is just an obstacle while his crew is trying to escape big mom is actually the main bad guy. this guy is just collateral damage therefore he's not as determined or confident in this battle. This battle is for protection purposes. I could be wrong but that's why it's just my opinion. ╮( ̄▽ ̄)╭

Don't worry peeps, either way this is definitely not the end of the manga, he's gonna win.

One thing I gotta say. Luffy is a plain goofball with anything so who's to say he doesn't have some stupid new power that is going to allow him to win? He never reads deeply into anything so he probably would not have some big dramatic end like Corazon ( ╥﹏╥). Anyway, before I cry, Luffy is probably thinking of something ridiculous and it should not be of worry. Besides it's Luffy; the man who will become the King of Pirates (and the main character of a predictable formula)!
What the hell would possess him to make face? That is NOT an "everything is going to be ok" face.That was sad and creepy all at once like corazon.... ╥﹏╥