Hello Everyone!
This can be accomplished in ACT 4. After completing the second phase of THE ABOMINATION boss battle, you will have to chase it and shot the weakspots as it is running away. Continue a little bit past that point and you will come to a more open room with multiple tendrils reaching from side-to-side. The Golden Skull collectible is on the right hand side tucked away at the end of where one of the tendrils meets the wall. Please see the attached video if you prefer visual representation. Good Luck!