Erza wins mid difficulty.
We can only speculate about this based on character feats and from their fights with similar opponents, so I'll start with their respective fights with the Spriggan 12: Ajeel and God Serena. While I agree that God Serena is significantly stronger than Ajeel, the latter was
compared to Brandish in terms of magic power, and her power is immense. We also know that the Spriggan 12 are
as strong as the strongest wizard in Ishgar, God Serena.
Let's start with Erza vs. Ajeel. Titania did fairly well on her own against the Desert King. Although she was overpowered in the end, she was able to make him use what appears to be his strongest spell, Sand World, implied by
Badd's comment on how he was forced to resort to it. Erza was only going to lose because Ajeel can move freely in the sandstorm and attack from anywhere; with Erza's vision hindered, the outcome was predictable. Had it not been for Erza's
quick reaction in using her Morning Star armor, she would've died.
4 Gods of Ishgar vs. God Serena. Despite the combined effort of 4 of the strongest wizards in Ishgar, God Serena
overpowered them quite easily and he wasn't fighting at his full potential. Granted, God Serena
admitted that they could pull off the win. However, as soon as he got
serious, the 4 Gods of Ishgar were biting the dust yet again...
If we are to believe that Jura's strongest spell, Rumbling Mt. Fuji, could potentially defeat Erza, we can easily compare its power to other highly damaging spells withstood by Titania, like
Jupiter,
Terra Clamare and even
Yagdo Rigora. I honestly don't see this spell granting Jura the win, as Erza as shown enough durability feats to withstand it.
Let me make it clear that I do
not believe for one second that Ajeel is quite on the same level as God Serena. However, current Jura doesn't stand a chance against current Erza.
Long story short, Erza wins "because she's Erza."
