I know most of you won't agree with me on this,but I will write this anyway.
Round 1- This is pretty easy.GMG Jura hellstomps GMG Natsu.Chances of Natsu surviving is out of the question
Now the difficult part...
Round 2 - LFDM Natsu has a no to slim chance of beating jura.Why?
We all know that natsu has a very high stamina and durability that might be close to laxus' own.He has been shown to tank most hits.if he could survive jellal's magic which was said to literally have the force of an actual meteor,I don't see how he can't tank jura's attacks,as mountain is pretty close to meteor.AND THIS HAPPENED WAY BEFORE GMG!!!
Besides,natsu has a good mind when it comes to fighting,also since no other restrictions are given except DF and LFDM,I will assume he uses his power of feelings here,which makes his fire a whole lot stronger.now combine that with lightning.
What do we get?A monster capable of bringing down mountains.Now, I am not saying that Jura is weak.He is the strongest human mage alive,so he is pretty strong.But even he will face a lot of difficulty in bringing down natsu.The match can only be judged(based on the restrictions given and the assumptions included) on how the two think and act during the fight.So Natsu won't go down that easy in round 2 but he doesn't have a great chance of winning against the veteran saint wizard.
If you guys have any corrections,pls feel free to do so,with PROPER REASONS ONLY!!!
P.S: I personally won't vote for natsu though,he only has a slim chance of beating jura in GMG based on other external factors and circumstances that may or may not occur