Nobody here is multi-continental with a single attack. Mercphobia was dropping water pillars all over the city. Each individual pillar was building scale in damage, and the damage was spread over the whole city. This is a city scaled attack. Ignia dropped his flames on the city. Now Ignia was holding back with that attack, because at full power Ignia would destroy at least a country. Nonetheless, this was a city scaled attack. Mercphobia dropped the rest of the ocean in an attempt to snuff out Ignia's flames. This caused portions of the city to flood. Again, this is at best a city scale attack. Elmina, a city, is still standing after these 3 attacks. Suggesting that everyone here is multi-continental when all the attacks combined could not even completely destroy a city is silly.
More importantly, Mercphobia lifting an ocean can't actually harm anyone. The water itself doesn't do any damage. Ignia was able to swim through it. Natsu was also able to swim through it, even before eating Mercphobia's flames. The only way Mercphobia is going to kill Ignia with that ocean is if he gets sucked in by the whirlpool and drowns in it, which isn't happening. Igneel has enough physical strength to brawl with Acnologia for 20 minutes, which means he would be able to brawl physically with Ignia for 20 minutes. Ignia could effortlessly swam through Mercphobia's ocean, so this shouldn't be a problem.
One more thing. Mercphobia's attacks don't scale to Ignia's. Ignia launched a single attack against the city of Elmina. Mercphobia tried to put out the flames of that attack and failed. Then Natsu ate the flames from that attack, and proceeded to defeat Mercphobia in 4 hits. If Ignia took that power of that attack and personally fought Mercphobia, Ignia would have oneshotted Mercphobia. If you think Mercphobia is stronger than Igneel, then Ignia can oneshot an opponent that Acnologia spent 20 minutes fighting and lost an arm against. That would put Ignia in a completely different league. If you think Natsu with Ignia's flames had 100% of Ignia's power, then that would mean Natsu is completely out of Acnologia's league as well.