The Elite Mantis Shrimp can be found in the Voltaic Hatchery, in the south east of Chersonese. Once inside, take the first right, then left, then right again down the path. You will approach a Lightning sphere which you can interact with to enter the Unknown Arena. However, you will need to have 5 Gold-Lich Mechanisms in your inventory before you can do so. To acquire them, defeat the nearby Golden Minions and Specters. Each one will yield a Gold-Lich Mechanism when looted.
Once you’re inside the arena, you will see the Elite Mantis Shrimp in the centre of the room. It won’t attack until you approach it further, so use this time to apply any buffs from potions and spells. You should also apply an ice rag or varnish to your weapon, as this is the only damage type which the boss is vulnerable to. As with all these Unknown Arena fights, you will need plenty of health and stamina potions to stay alive and rescue you from sticky situations.
The Elite Mantis Shrimp is almost identical to other regular shrimp mobs in terms of its attack capabilities. The only difference is that it sometimes produces two balls of Lightning damage instead of one, which you will need to avoid in the same way that you do with regular shrimp mobs. The balls of Lightning can track you, so it is important to move as soon as you see the animation. The animation for this attack is signalled by the shrimp raising its claw in the air.
In addition to the balls of Lightning damage, you will need to avoid the explosive charges which the Elite Mantis Shrimp emits when you enter within melee range. These are emitted in the same way of the regular shrimp, but they can be followed by one or two explosions quickly after. While one explosion is not enough to knock you down, two or three will almost always knock you down without high impact resistance. You should therefore seek to avoid these attacks as soon as they are initiated.
Ultimately, the mechanics of this fight are rather simple. Keep an eye out for the lightning attack animations, and stay behind the Elite Mantis Shrimp as much as possible, avioding the melee attacks and making it more difficult for the Lightning attacks to hit you. The boss heals itself 1.5 health per second, so it is important to maintain pressure by chaining attacks. The use of stamina potions rather than waiting to regenerate is therefore advised. Just be careful not to run out too often, as you will be unable to evade incoming Lightning attacks without sufficient stamina.