Hey there!
It really depends on what part of the move you're having trouble with. There can be a lot of factors, and depending on your more foundational moves, certain strengthening would do very little, and others could do a lot for you.
So, to start out: Can you do a one handed fireman spin? How about a one handed front hook spin? One handed Chair spin? In order to start working on the reverse grab, I would very seriously suggest being very comfortable with a one handed chair spin, on both sides, without allowing your shoulder to slide up toward your ear AT ALL. Your shoulder should never slide up toward your ear.
This should engage the larger muscles of your side and back (lats), as well as many of the larger muscles to support your shoulder stabilizers. If you aren't able to do a one handed chair without losing integrity in the shoulder, then your weight will end up on those little teeny shoulder stabilizers, and you'll end up being injured very quickly... not to mention finding the reverse grab spin quite uncomfortable, if not impossible.
Next, getting your hips forward and around the pole requires quite a lot of core strength, especially if you are swinging your legs fairly far out (which is better to gain momentum for the spin, but is much harder, so should be considered a progression thing). Your back, obliques, and lower abs should all be working here to keep your hips up, your back supported, and lift your legs (that way your hip flexors don't have to do that job and then get mad about it). This can be one of the hardest things about this move, because you really need to lift that leading hip and twist your body around the pole. Spins like the cradle and tuck can be great for getting your hips up a little more, and for getting used to that position. The corkscrew spin can also be a great one for getting used to the movement.
Finally, the technique to a good reverse grab can be a little illusive, and take a lot of practice. The swing out at the beginning is very important for having appropriate momentum for this spin, and even just working out how to transfer the movement and your body together can be hard. Veena places this spin as "Advanced" for good reasons!
So, if you feel like you've got most of the foundational moves and spins behind this one really, really solid, then just keep practicing.
If you don't have them solid, though, then go and give those a shot for a while. They really are beautiful spins and moves that you can add to your repertoire, and are great for strengthening your body!