This one can be a bit annoying I found the best way to to do this is to get up to the knife and get close to the hole, now the side you want to be on is so the knife is behind the tentacle.
Now here comes the part that can be a bit hard to do:
Get your tentacle close to the hole so you can see the other side through the hole and tap 3 times as fast as you can in the middle of the hole (so technically you are trying to hit the ground you can see through the hole).
Doing so should get you the achievement but it may not work first time so continue to try (shouldn't be too hard)
Let me know in comments if you have any issues regarding this :D