I've highlighted the QTE box on this image. The orange box with the bar in the middle.
If you're playing on PC and haven't reassigned keys, you should need to keep clicking the left mouse button, fairly quickly. You should see the bar fill up. When the bar fills, you'll come off the tentacle, and be standing on the ground. Is that happening?
If this just isn't working, there's a few things you can do. Try dropping the screen resolution and use windowed mode just for this section. If you're really stuck, you can upload your savegame from just before the fight to a hosting site like mediafire. Post the link here, and someone can play past this point for you.