So, this came to me. As far as I know, Logia's have the ability to become, create, and control their element. However, they have to activate the ability to become their element. For example, Crocodile had to activate his ability to become sand when Luffy threw a few punches in their first round. It was instant. So, if you're fast enough, and can deliver enough damage within the first few seconds of a fight before the Logia can activate their ability, then you would have a chance of winning. This could explain how Gear 2nd is all Luffy needs to defeat a few Logias. For example, we'll use Aokiji (even though I don't think Luffy can beat Aokiji now ). If Luffy starts off immediatley with Jet Gattling in Gear 2nd, that may be fast enough for Aokiji to not realize what happened, and not be able to concentrate and use his ability. And in the split second Aokiji is falling to the ground, Luffy could come with a Jet Pistol, or any other attack. ( As a note, after the second attack of Gear 2nd from Luffy, I'm pretty sure Aokiji would catch on and win pretty easily ). Anyway, what I'm getting at is, Speed may be the key to defeating Logias.