Natsu easily. Volcanion's strongest moves are Fire type moves and Natsu is immune to fire. Volcanion doesn't have knowledge about that so if it uses any fire type attack Natsu eats it and becomes stronger.
Water type moves are Volcanion's second strongest, but Natsu has shown the ability to evaporate water, so it's safe to say that Natsu can deal with those.
Natsu is a trained fighter who has fought against numerous foes. So experience is with him in this case. Volcanion can't say the same though. Because of his combat training and his enhanced senses Natsu will most likely most of Volcanion's attacks easily, even with equalized speed. Volcanion is unexperienced in fighting and that will be his downfall in this match.
This is actually pretty difficult. Natsu's fire attacks ain't doing crud to Volcanion, and Vocanion's Fire attacks are doing even less, assuming Natsu eats them. However, Steam Eruption is Volcanion's most powerful move which is Water Type, yet Lightning Flame Dragon Mode is gonna screw over Volcanion. Ultimately, the winner should be whoever has the higher AP, seeing how both are evenly matched in many categories. Volcanion would likely figure out that his fire isn't working due to Flash Fire being a thing in Pokemon.
Flash Fire is a thing is pokemon, but there is no real proof that Volcanion knows about it. Trainers with great experience in battling don't know every pokemon's ability and pokemon tend to battle way better when they have a trainer ordering them around than when they fight alone. Also while Natsu's attacks are enhanced by fire, Natsu is a great hand to hand combatant so his punches and kicks are gonna do some heavy damage. If he cloacs his attacks with fire they will do more damage, even when Volcanion heavily resists fire. Also, one or two fire attacks from volcanion are enough to neutralize it's resistance against fire and make matters even worse for it.
Natsu would lose this one because he's only given Lightning Flame Dragon Mode and he's not mountain level until he goes into his Flame Dragon King Mode he would only scratch Volcanion. Natsu may have a technique advantage but Volcanion has many different types of moves Mostly Water and Natsu can't evaporate that much until he's in FDK mode.
^ Since he's put against the Mountain level Volcanion, I say this is Fire Dragon King Mode Natsu.
As for me, Natsu hasn't used FDK mode AND LFD mode at the same time, right? Haven't seen Natsu used LFD since 2nd timeskip. And frankly the most prominent water user in FT is no match for Natsu (sorry Juvia ;_;) while Aquarius is questionable, so I have no idea who would win.
If its Fire Dragon King Mode Natsu then yes Natsu should be able to completely crush Volcanion he evaporated an entire lake he can just vaporize all of the Water in Volcanion's tank then he'll be left empty and defenseless GG.
Natsu, his FDKM evaporated a lake with it's basest move, if he switches modes on the fly or just stay in LFDM(his base was hot enough to melt the coloseum so Im sure his flames in LFDM would be hot enough) it would give him the advantage maybe use Exploding Lightning Blade as a finisher