This is an easily missable achievement. First off, you'll want to be aware of how to craft the trap. The diagram is boughten from Cedric outside of Flotsam. The only missable ingredient is the Iron Frame (found in the chest at the end of Loredo's garden, but ONLY during the quest "Indecent Proposal". If you go back after finishing the quest, you won't find anything). The other ingredients are as follows:
- 1x Iron Frame
- 1x Iron ore
- 1x Silver ore
- 2x oil
Take the ingredients to Cedric or the dwarf and have him craft the trap for you. Then, during the fight with the kayran, run to the left side, towards the outline of the trap. Press the button prompt to deploy the trap, and then bait the attack into the trap. The trap will cut off the tentacle by itself, and the achievement will unlock.