You just have to fight the monster. As far as I remember there were no other consequences. Wanted to check it out on youtube but couldn't find a a video from someone who didn't throw the baby into the oven once you advanced that far in the quest, which should tell a lot about how onesided the quest is.
The same applies to the king/queen choice. You will find barely anybody who supported Hjalmar. Though why should anybody do this?
The whole choice is basically tradition vs progression. The problem is that keeping the tradition is presented in the worst possible light.
- during their quests Cerys achieved her goal by curing the Jarl, no matter what you do. Hjalmar on the other hand looses almost his whole crew while slaying the ice giant.
- Cerys is presented as cunning and calm, while Hjalmar is shown as hot headed, impulsive and a bit incapable
- Choosing Cerys will lead you to the people responsible for the bears attacks, while choosing Hjalmar will only help you to find the "tools" for the attack
- If you choose Hjalmar he will start a war against Nilfgaard, which will lead to the Skellige people being soaked in blood. Cerys will keep peace and Skellige will progress
- On top of that there is a place of power, which is only accessible if you choose Cerys.
There is no reason to choose Hjalmar. I was surprised they didn't show us a developer note, that we should support Cerys.
It's a hit and miss regarding the choices in Witcher 3. Some are difficult but too many are clearly good vs bad ones. I definitely prefer the previous games in this regard.