Just a 3/8” ratchet and a good ol’ hingggfffph. I would stay clear of heat in this instance. Maybe a few drops of penetrating oil to see why that gets you and then. You can also try the tighten it a touch to see if you can get things moving. An impact would do alright, but I didn’t have one until recently. A whack on the end of the ratchet with mallet is essentially the same thing while being slightly less aggressive. Jerk they call that, 3rd derivative of position…#PhysicsShit