Invading the Elemental Plane of Fire

So the Elemental planes have been locked in combat for ages. For the climax of an upcoming campaign, I need to figure out how Water would invade Fire. Lets do the math.

The Gate spell says it can link two portals across planes and can be anywhere from 5 to 20 ft in diameter. 20ft is 6096mm. The pressure of the elemental plane of water is no greater than "being under a few feet of water" so lets say, ten. Pressure under 10ft of water is 0.298907.

The gate spell also states "Deities and other planar rulers can prevent portals created by this spell from opening in their presence or anywhere within their domains." So, rather than cast the spell into the Palace, water forces would have to highjack some of the "Gate" spells around the edge of the city. 

Roughly 14 million liters per minute! Or 6.5 olympic swimming pools a minute. I hope the sultan has flood insurance.

Wall of Water says "You can make the wall up to 30 feet long, 10 feet high, and 1 foot thick" so 300 divided by 5 and 5 again, is 12, so 12 5x5 squares of water. that's 60ft of water to swim through, which they could utilize as siege ladders to scale walls.

So strike groups could attempt to overthrow defenses. There would be a brief window when these portals could be reached safely, but the longer they stay open, the more favorable ground exists for the forces of Water.

So the key would be getting multiple high level mages, into the City of Brass and simultaneously taking control of several gates.