Wont Gengar be automatically immune to speed blitzing? Without Fire magic, Natsu's physical attacks wouldnt do a thing at all against Gengar due to intangibility.
Also is this a composite Gengar or a specific one? Because Gengar is capable of hiding in ppl's shadows so it has perfect stealth and at the same time can either cause curses or absorb someones life force.
Also since every attack Gengar can use is allowed here, it has abilities like Mind Control/Manipulation via Hyponsis, Nightmare or Confuse Ray, Curse infliciton via Curse, Possible Durability Negation via Shadow Punch (since its a punch made out of intanigible shadows), Mind crushing via Dream Eater, Maybe Destiny Manipulation via Destiny Bond, Barrier Creation via Protect, Psychic Manipulation, Weather Manipulation via Sunny Day/Rain Dance, Elemental Manipulation, Energy Manipulation, Resistance to mind manipulation via Sleep Talk, Reality Warping + Pocket Dimension creation via Trick Room and Wonder Room and they all get a huge boost via Mega Evolution
Also, IIRC, there was a gastly who was not only able to create illusions, like pokemon and fuse them together, but they could also physically attack real things so thats a reality warping feat right there. And obviously Gengar>>>Gastly so you get the idea
Even without Mega Evolution, I dont see how Natsu as any chance at all against hax like what Gengar can do