Spoiler: Just reached the red heart in the Apex but i can't interact with it in any way.
Had to go back to town.
Did you try to hit it with a weapon?
I first attacked it from a distance with a Gate Keeper spell: The spell fired but no damage was dealt and it didn't start the battle either.
I then tried to "Look"/Attack with a melee weapon/Checked for weaknesses...
Hope it helps, will get back to it and try to just get closer in case their is a specific trigger i messed up with the GK spell.