To unlock this achievement, you'll need to have the Kraken's Claw (Sword) already in your deck. In order to get that sword, you need to have defeated the Kraken once already and received the token from doing so.
You'll also need to have completed all The Land Locked Lubber encounters and have The Kraken Unleashed in your deck (obviously).
It's a good idea to have Herne's Antlers in your deck. Having both this and the sword equipped should give you two lots of x2 (4x?) damage against Lizardmen and also allow you cut through their shields. This makes the battle against the Kraken a lot easier, that is, if you are lucky enough to get one or both of these items.
To make things easier, you should put equipment in your deck that makes drawing equipment more likely. You want to increase your chances of drawing Herne's Antlers and the sword. The Ring of Reward is one of the cards that will be useful for this purpose (Honestly, if you have it, this should always be in your deck anyway).
As for the battle:
- Stand on the metal grates at the start of each wave to avoid being damaged by the tentacle that bursts through the floor (and spawns two Lizardmen).
- Prioritise the Lizardmen over the tentacle with the weak joint if you can. If you don't take it out in time, it'll attack again after a short while and give you another chance. (Instead of starting the wave again)
- Be wary of the electrifying tentacle when fighting the Lizardmen. Try and avoid standing on the metal grates this time.
- It's still possible to take out the Kraken head with Lizardmen alive, but difficult. You have more time than you need, so dodge/counter attack and/or stun them. Resume attacking the Kraken ASAP.
- Lastly, remember to avoid the King of Scales's (I think it is him) laser-shield attacks and counter/dodge everything as always.
Once you defeat the Kraken, you'll receive the mask again. Equip it and make sure you have the sword equipped to unlock the achievement.