Whichever way you look at it, his FoE kicked in after a long period of fighting and him getting resurrected from the dead. Why would he use FoE on someone from the guild when it's the guild fire??No it didn't. That's silly. It just takes Natsu a certain emotional state to activate. Just because the fight lasted 15 or so chapters doesn't mean it takes 15 chapters for that specific attack to charge up. That's like saying that it took Irene 11 chapters to activate her dragon form and swing her claws because she started fighting Erza in chapter 507 and took until 518 to enter her dragon form.
Could Irene and Laxus beat Natsu before he works up the emotions necessary to overpower them? Possibly. But those are scenarios. I'm just interested in who is the most powerful.
His DF on the other hand maybe, because it's now easier for him to access as we've seen since Tartaros.

