Probably Luffy using CoC IMO.
So far Loki hasn't really registered Luffy as much of a threat.
If Loki says he's going to be King of the Pirates, and tells Luffy he can't become King, I can definitely see Luffy revealing his full CoC level, like Shanks did, and knocking out the followers, or perhaps the Giant Beasts.
Only think Luffy's Haki hasn't been able to damage in the series so far is Seraphim and Warcury (and maybe Kizaru depending on how much Kizaru held back against Luffy), and I have a really hard time putting Loki over Warcury when two Giants with a combined 3.6 Billion Bounty couldn't damage him.
Loki's probably a Saturn-level threat with the idea that he's the strongest in the world, IMO: dangerous, but not able to fuck with Luffy in Gear 5.