This is the battle between Link and Morpha: Giant Aquatic Amoeba, from The Legend of Zelda: Ocarina of Time.
Link is using the Longshot, the item from the Water Temple; the Zora tunic, which allows him to breath underwater and the Iron boots with which you can walk underwater.
As allways I've tried to be as loyal as posible to the design of the original game from Nintendo 64 rather than using oficial ilustration as reference.
I must say that, in my opinion, this and barinade are the worst bosses of ocarina of time, specially because after beating all the water temple, which was so frustrating due to the way you had to equip the iron boots, this boss was not rewarding enough. However, it has been a big technicall challenge because of the hard perspective and the water effects. Hope you enjoy looking at it as much as i did painting it.