Short YouTube video made by Draisaitl for those that don't want to read below!
Alchemical Hydra
Health: 1100
Level: 426
Max Hit: 17+17 (default) / 26+26 (empowered)
Aggressive: Yes
Poison: Yes
Weakness: Stab, ranged, dragonbane weapons
Attack style: Ranged and magic
Instanced: Yes
Slayer Master: Konar
Requirements: 95 Slayer
Location: Karuulm Slayer Dungeon
Recommended: Completionist monkey - acts as boots of stone, boots of brimstone or granite boots
(Thank you @Draisaitl for reminding me)
Getting There
Note: Be sure to wear Granite boots, Boots or stone or Brimstone boots to avoid reoccurring damage in the dungeon!
Noteable Loots
Strategy
it's rather simple to kill The Alchemical Hydra.
Lure it to the vents as seen below and avoid any poison/flames.
Green form -> red vent
Blue form -> green vent
Red form -> blue vent
Gray form -> vents = useless
Vents (Chemical Pools)
If you lure it over the wrong vent, it will have 50% increased damage towards you.
This damage increase is permanently on when the Hydra reaches the penultimate (gray) form.
Hydra attacks with Ranged and Magic, switching styles in a predictable pattern every 3 attacks.
The Hydra will change phases during the fight as follows:
100% HP - Green phase
Hits up to 17, and uses a special poison attack. (50% more damage if on wrong vent)
75% HP - Blue phase
Hits up to 35, and uses a special lightning attack. (50% more damage if on wrong vent)
50% HP - Red phase
Hits up to 35, and uses a special fire wall attack. (50% more damage if on wrong vent)
25% HP - Gray phase
Deals 50% more damage (up to 52) and has a 75% damage reduction which cannot be removed.
It is important to note that the Hydra has a 75% damage reduction by default, which can only be removed by luring it over one of the vents in the room (opposite of its current colour/style).
Poison
In this phase, the hydra appears green and should be lured to the red chemical vent.
The Alchemical Hydra will launch 4 or 5 poison blobs around the player's location after the first three attacks. These blobs target the player in a 3x3 area and continuing to stand on them will deal up to 12 damage per tick. These blobs have a short travel time, so if the player begins running as they are fired, it is possible to avoid their damage completely. The poison is fairly weak, but it is recommended to clear its aftereffects immediately.
Once the Hydra reaches 75% of its health, the bottom-right heads falls off and it changes its carapace to blue.
Lightning
Now with a blue appearance, the hydra should be lured to the green chemical vent.
Throughout the phase, the Hydra can launch four electrical projectiles around the room. The lightning always spawns in the same areas, but the order in which they appear varies each kill. The four pieces of lightning converge on the player's location. Getting hit by them will bind the player and deal up to 20 damage. It is not possible to tile skip to avoid them. Starting near the middle of the room and looping around one of the last lightning spawns, to escape the box they form, is the easiest way to avoid their damage. Players may also hug the northern wall and quickly move their way west while damaging the Hydra, which works especially well with a twisted bow due to its attack range and ability to avoid the lightning with ease.
Once the Hydra reaches 50% of its health, the bottom-left head falls off and it changes its carapace to red.
Flame
Now with a red appearance, the hydra should be lured to the blue chemical vent.
Throughout the phase, the Hydra will occasionally walk to the center of the chamber and face the direction the player is at. Once it reaches the center, the player is stunned, but they can still attack the Hydra. Upon reaching the center, it breathes 5x5 layer of fire to the player's sides based on their location before staring down the middle and launching a tracking fire. This tracking fire follows the player's movement, and travels significantly faster the further the player is (or if they start running). Getting hit by any of the fires will result in the player taking up to 20 damage, followed by a subsequent bleed of 5 damage for five hits. Moving onto another fire resets the bleed. Fires may be tile skipped, though it's generally advised against doing so. The Hydra does not attack until four seconds after the tracking fire is sent out.
Once the Hydra reaches 25% of its health, it changes its carapace to gray.
Enraged
There is no counter to the gray carapace. It does not need to be lured as its damage reduction ability does not reset.
The Hydra starts off this phase by attacking with the combat style opposite of what it used in the previous phase; if the previous phase ended with a magic attack, then it starts with ranged and vice versa. In addition, the Hydra's damage from its auto-attacks drastically increase, and changes attack styles after every attack it performs, regardless if it hits or not.
It also uses the poison attack from phase 1, though it occasionally sends one poison blob instead (on the tile the player is on). During this final phase the poison attack will always happen after 3 normal hydra attacks, then again after another 9. Continually pray flick until the Hydra is killed. It is recommended to keep prayer points up to prevent the possibility of the Hydra landing a hit on the player. If the Hydra is killed just as it sends an attack, then it will deal no damage.
Recommended Equipment
Will keep it updated to make it Alora-appropriate!
Credits: 07 Wiki, @Draisaitl, Alora & @Omicron
None of this is my doing. I simply put it into a guide for everyone to see.