Before you even begin to fight the scourge of the Divine Beast Ruta, here’s a bizarrely important thing to know: You can straight up leave the room in which you fight Waterblight Ganon. If you’re ever low on hearts or you want to regroup, just walk up the ramp you took to get in, and you’ll be safe. ¯\_(ツ)_/¯
Also, before you begin, make sure you have food or elixirs that increases you defenses. Think of food in Breath of the Wild as temporary armor. If you eat something that boosts your defenses for a couple of minutes, that'll be plenty to survive the fight. It can turn one-hit kills into survivable strikes.
Waterblight Ganon phase 1
Waterblight Ganon's fighting style changes depending on how close (or far) you are from him. Our preference is that you keep your distance and rely on arrows for your primary attack.
Waterblight Ganon telegraphs his attacks — mostly a thrust, a spear throw and if you get close enough, an attack that shakes the ground beneath him (browse the gallery above to see them all). Putting space between you and him gives you more time to react and makes those attacks easier to avoid.
After chipping away at about 20 percent of his health, Waterblight Ganon will fall to the ground. Run to the monster, equip your most powerful weapon and attack.
Repeat the process until he loses about half of his health, at which point the second part of the fight begins.
Waterblight Ganon phase 2
When Waterblight Ganon’s health dips below 50 percent, the second phase of the fight begins. Instead of having the freedom to move around the area, you’re confined to four blocks, and you have to swim between each.
Waterblight Ganon will cling to the ceiling to attack from a distance. Get to the platform farthest from him and launch your attacks. Use your most powerful bow and whatever arrows you have — though of course arrows with status effects will deal more damage. (It seems that shock arrows are the most deadly.)
Watch out for his spear lunging attack, which you can dodge or jump off the platform to avoid. He’s also fond of throwing his spear at you, and you can avoid that in the same way.
Waterblight Ganon will also use the same tricks that Divine Beast Vah Ruta used when you were trying to break into this mechanical elephant. Equip cryonis and destroy any ice blocks he throws your way. If you miss, you can dodge at least some of the remaining ice blocks while swimming to another platform.
Watch your health, keep eating to increase your hearts, and keep firing arrows from a distance until he falls.
The aftermath
Defeat Waterblight Ganon, and you’ll receive a heart container, which raises your maximum number of hearts.
Interact with the main control unit to leave Divine Beast Vah Ruta. Note, however, that when you leave, you’re gone for good. See above if you’ve missed or skipped any treasure chests.
Mipha’s Grace
Interact with the terminal, and you’ll meet Mipha, who will give you Mipha’s Grace — a literally life-saving mechanic that resurrects you when you die, restores your hearts and grants you several extra, temporary hearts. It’s kind of the best.
You can track its readiness with a glance at the elephant icon at the upper left of your screen. When the elephant icon is white, it’s ready to use. When it’s hollow, that means it’s in the cool down period after you use Mipha’s Grace. That lasts several minutes.
Completing the Divine Beast Vah Ruta quest and getting the Lightscale Trident
Return to King Dorephan in Zora’s Domain, speak to him and complete the Divine Beast Vah Ruta quest.
After the cutscene ends, open the treasure chest to get the Lightscale Trident weapon. As you’d expect, this weapon can break. Unlike other weapons, though, you can repair and reforge this weapon. Just speak to Dento in the Zora’s Domain shop.