For most players, Marlin was one of the most annoying bosses because, he punished you for being good at the game. But in the exception of Maru who had the ability to decide which ability he wanted to be shot with, he still decided to choose the arc because, if you kill Marlin you get whatever item he shot out.
The ark was the most valuable because, it had the ability to erase anything or anyone and it has 5 uses and under the perfect conditions you could even get 15 uses. The boss fight began as expected with the long boring speech nobody likes hearing. After the speech they began fighting.
The condition necessary for him to get 15 uses was for him to not cheese the boss. Normally players would cheese the boss causing it to enter stage 2 and start spamming the arc which wastes it's uses.
But, if you fight it with clean and pure swordmanship alone, it Marlin will fight back accordingly and not use the ark.
Although fighting this way takes longer, it's the most effective way to fight it. At the end of the fight, Maru kills it with perfect health granting him the 15 use ark and an achievement called "Stainless victory" and the god pendant which gives him the ability to fly or rather levitate very high lasting for 7 seconds and with a cooldown of 1 minute.