Serena gets one shot in the first second.
DF Natsu can at least put up a fight.
They both can eat each other's flames. If anything, this gives Natsu a relative advantage in combat stats because this allows him to maintain DF for longer, plus can power him up a bit. While Ignia's flame is capable of damaging Natsu while vice versa not true (yet), because Ignia has stronger flames.
So, "flames" into account, nothing eventually changes IMO.
If they engage in pure physical combat, Ignia wins mid diff
If they use all in their arsenal, Ignia still wins mid diff.
All things considered, Ignia wins mid diff.
The only chance in hell that Natsu and Serena stand a chance in surviving longer than thirty seconds is if they both team up and somehow manage to eat some of Ignia's fire. Assuming they do this, Ignia low-mid diffs the duo. If Natsu nor Serena manage to eat any of the DG's fire, it's a stomp in the Dragons favor.
It's going to be a mid diff either way.
DF Natsu should be able to compete in pure stats. Ignia's flames can give him the stamina needed to even maintain that combat stats.
