Waterblight Ganon is a boss in Breath of the Wild. It was created by Calamity Ganon to corrupt the Divine Beast Vah Ruta, eventually serving as boss of the dungeon when Link came to free it of Ganon's control 100 years later. If Link doesn't fight Waterblight Ganon inside of Vah Ruta, he will later encounter the boss in the Sanctum inside of Hyrule Castle.
During the first phase of the battle, Waterblight Ganon will wield a massive spear and will attack you with it. The most risk-reward method of battling it is to wait for it to attack and then try to time a Perfect Dodge on it by hopping away at the last second, followed by a Flurry Rush where you can deliver a number of attacks in a short period of time. The much safer route is to keep your distance and use projectiles such as arrows, or even just tossing your weapons out at Waterblight Ganon.
After Waterblight Ganon has lost half its health, this will trigger the second phase of the battle. The room will flood with water and only four platforms remain. Waterblight Ganon will create a block of ice and launch it towards you. The block can be disintegrated by using the Cryonis ability, so be sure to keep that handy, or frozen with Stasis. As the battle progresses, Waterblight Ganon will begin to shoot three ice blocks out at a time, and then five ice blocks. These can be quite damaging, so be sure to have Cryonis readily available.
After you get rid of the ice blocks, there is an opening where you can shoot Waterblight Ganon with an arrow. After successfully hitting it, Waterblight Ganon will move to a different platform. It is best to just stay put, unless it moves to the platform close enough to where you are standing. Arrows with an elemental effect, such as Ice or Shock arrows, will often stun it long enough for you to get over and wail on it. Just repeat this process over and over until its health comes down. Occasionally Waterblight Ganon will use its spear to attack, at which point, just do your best to avoid it. If you can time a Perfect Dodge, this is another opportunity to get in a load of damage.
If you do run out of arrows, then Perfect Dodges are about your only method of dealing damage. You can also freeze one of its ice blocks with Stasis, and then hit it back at it. You can damage it with bombs, but it is hard to get close to it.