Feats say more then words, which only was said for "hype".
You said about reason, ofc it has it, it is named "plot".
Natsu is simply too strong (after Alvarez arc) to face such no-names like these DE. All current events with him is just a mere charade. Similar i can say about Erza.
You said about water and why Natsu failed to boil it, as I see with making base of explanation on physics laws, right? Correct me, if I took you wrong.
If I correctly understand you, then it's hilarious.
Physics & chemical attributes ofc play their respective role, but don't forget about pure magic power and difference between "attacker" and "defender" magic power.
I strongly believe Natsu has magic power at least to boil water.
By the way, why I consider as hilarious such type of explanations?
Because it's double standards again.
1. Natsu failed to boil water - it's ok, he was underwater, it's psychic. (Rule for 1st type)
2.
Skullion turned water into ash - how he great! (Rule for 2nd type)
If I didn't forget all physics from school, water is H20 or H-O-H.
2H20 (liquid) x T -> 2H2 (gas) + O2 (gas).
As wikipedia says
"Ash or ashes are the solid remains of fires. Specifically, it refers to all non-aqueous, non-gaseous residues that remain after something is burned".
Usually it's quartz, hematite, magnetite and various calcium silicates.
Now rhetoric question - if someone explains case with Natsu and water in sequel (only with physics laws), should same person consider Skullion case with turning water into ash as purest BS ?!