Yeah, I just read up on Ace's profile since I'm not familiar with him, and I gotta say he takes this. I've read the Fairy Tail manga and watched the anime, and Natsu has little to no hand to hand combat skill against characters around his level because he normally spams his fire attacks until they overwhelm him where he eventually gets a nakama power up to beat them.
If speed is equalized and Natsu realizes that his nigh on only form of offense is useless, he's basically screwed because Ace is stated to be an expert hand to hand combatant, something I can tell from Experience that NAtsu is not. Especially since Ace can use a Haki enhanced knife along with his normal hand to hand combat, proving that he has at least enough experience to be proficent with a weapon, something that again, Natsu is not. I don't even think Natsu has experience canonically fighitng short-mid range weapon users, so combating Ace with a Haki enhanced knife isn't going to be something he will be able to handle very well.
And as for the LFD mode, I doubt that will mean much to be honest since the attacks still seem to be primarily fire. Sure the Lightning stuff will be some form of problem, but uh, unless Natsu can use completely lightning attacks, it won't honestly mean anything since Natsu at his peak seems to be 7-B, which according to what i read on Ace's profile is the low end of his Tier. I doubt a peak 7-B using/trying a new attack style (pure lightning) to get around an opponent's immunity would be able to stop a 'at least Low 7-B/7-B' before they can retaliate, but I might be wrong.
TL;DR: Ace has way too much combat skill and experience for NAtsu to handle, especially since NAtsu's primary fighting style is fire spam until Nakama power up
Side note: Why is NAtsu, a fire user that can EAT fire and recover with it only being put against other fire users? I've seen him put against Ace, Sabo, Charizard, etc. Sad part is, using their physical skill and in charizard's case, non fire abilities, they all beat NAtsu with mid-diff at most.
With speed equal and "natsu at his peak" which is with Igneel's power after 2nd time-skip, it would likely be a stalemate since Natsu would likely hurt Ace via eating and lightning before he starts running out of magic power and Ace's only means of harming Natsu is with his knife and fists.
