How to Kill Green Jellyfish
They should be vulnerable to magical attacks!
The spells you learn throughout the story and pull into an extra hotbar above the other two hotbars (tools etc). You can use the spell you have selected with right click.
And just in case other people have this dum-dum moment and don’t know how to toggle / swap the active spell on the hotbar, just left click the spell icon on the hotbar. You can swap them with mouse wheel aswell.