Gatekeeper Kaleida is the final boss of Deepstrike Mines. He is also by far the hardest.
Tactics
Gatekeeper Kaleida throughout the fight will transition between two elemental attributes: Earth and Fire. His elemental attribution can be seen as a buff and he has different abilties depending on which element he is attuned to. He also will toss a crystal at a random player, either earth or fire depending on his current state that does a small AoE knock-back and some damage. Gatekeeper Kaleida will change his elemental affinity shortly after the second crystal is tossed out, and continue to change back and forth every two crystals.
The Elemental buff can be removed with buff removal skills like Purge.
The tank should position the boss where he is standing with the boss faced towards the wall and DPS behind him. Melee should be at max hit box and ranged should spread out, but do not use the ramps on the sides of the room, stay in the pit.
Earth Attuned
When Gatekeeper Kaleida is under the affects of the earth buff he will toss earth crystals and will cast a small earth AoE around himself. The earth AoE damage is not signficant and can be easily healed through, or melee may choose to run out as he is casting to try and save the healers mana. DPS need to watch for when he tosses out a crystal at a random player. You can tell who is going to be hit by watching for a small yellow-green orb flying through the air at a player. He may target anyone, including the tank. The orb will follow you so it is impossible to avoid getting hit by it.
After the orb hits the player a large yellow crystal will spawn at that spot. For every two seconds the crystal is alive, the boss will heal himself for 1% of his health. All DPS, ranged and melee alike needs to immediately switch to the crystal and burn it down as fast as possible. The longer the crystal is alive the longer the fight will take, and if DPS is slow he may heal up to full health. After two crystals the boss will change elements.
Fire Attuned
Fire phase is the burn phase of the fight. When Gatekeeper Kaleida is under the affects of the fire buff he will toss fire crystals and will cast a fire AoE around himself. Melee need to avoid this at all costs, when you see the cast bar run out. He will also be tossing fire crystals instead of earth crystals. They have the same animation as the earth crystals when in transit to a player only they are red in color.
These crystals do not need to be DPS'd down, they will explode on their own. If you see one near you run away from it. The closer you are to it, the more damage you will take when it explodes. If you have a shield ability or a way to mitigate damage you should use it in this phase. If a crystal lands on a tank, the tank should blow a cool-down and healers need to be ready for the big hit. After two crystals are tossed, he will do one last fire nova AoE before he transitions; Remember to run out of it melee.
Gatekeeper Kaleida will continue to transition back and forth every two crystals, so remember to anticipate what you need to do and try to avoid and absorb as much damage as possible as this can be an extremely long fight.