This is a Composite Natsu. All of them will be one-shotted. So he can use all the forms he has used up to now. Natsu's Ignia turned Mercophobia(natural born dragon) into a human with his flames. The only one who would benefit from this will Irene provided she isn't already ashed. I just feel sorry for the rest. Erza will be turned into a lesser human along with his plans.
Ok, I was thinking about Natsu as his own ressource, no Ignia's boost or Ignir's one shot ultimate Dragon King power. In his own ressource, all of the 3 are clearly above (Gildarts, Irene and August, solo), even with Zeleph as partner.
If we can count Ignia's boost, Natsu were on par or slightly above Mercphobia who were half power. So he is above them in one on one but vs the 3 or 4 together, I think this isn't enough. Irene can boost allies with incredible enchants if necesary. Still, Gildarts, Irene and August are, until now, the strongest wizards in the Manga, with Erza under conditions (PoF).
But taking account of external power up is not fair. It is not his own magic power and can't use it again, like his true Ignir's flames.