No Pressure is a grandmaster combat achievement which requires the player to "Kill the Alchemical Hydra using only Dharok's Greataxe as a weapon whilst having no more than 10 Hitpoints throughout the entire fight."
Enter the Alchemical Hydra's chamber praying either Protect from Missiles or Protect from Magic. If it attacks with the opposite style, teleport out immediately and try again.
The rest of the fight is a typical melee fight; see Alchemical Hydra/Strategies. Due to the low damage per second afforded by Dharok's equipment, it is likely that the player will face multiple special attacks per phase. This occurs after the ninth regular attack following the first special attack. Make sure to start moving in advance of the second poison blob attacks during the first and fourth phases.