After you have chopped 3 of the tentacles it knocks the statue down, use the chunck of rubble thats lands in front of it for cover then move forward between the flying rocks and make your way up the impromptu bridge to finish it off.
You cant trap the front 2 tentacles either, only the 2 that are on each side of its body. Start on the left 2, cast yrden on the ground then watch the tentacle, once it raises up, raise your guard and then roll left as it starts to come down, this should trap it. Then move up on the outer side of it and hack away. Rinse and repeat for far left and center right.
After you have chopped the 3rd tentacle move far left and you will get swiped and hang onto a tentacle, hit the button it shows to stab the tentacle then when it drops follow the cover system I mentioned above.