Fix one end of a barbell into the ground (e.g. into a corner of a room) then put plates on the other end. This setup is called a "landmine" for some reason.
The benefit of this is that the arc of movement is fixed to the spherical plane so you don't need so much control of the movement as with dumbbells and can thus lift more weight and be more consistent in your form. In some cases I can do more reps with twice as much weight than with a dumbbell variation of the same exercise.
Picrel is the exercise I was talking about in my previous post. I tried spellcasters with dumbbells like anon recommended, but I have to say that I could still feel my core struggle more with this landmine exercise than with dumbbell spellcasters simply because I could work with more weight.