This is flare connector, where 2 pieces are sealed by tapered metal. That comes with pretty high torque, what combined with rust makes for hard removal.
As 260 mention, good flare wrenches can be crucial and the smaller hex on metal side is more prone to stripping.
If you can disconnect the other end easier, I would slide box wrench over it. Looks like you are for hose replacing anyway, so no problem to cut it and use box as well.