When I used to help with CPL classes, when a student obviously had grip issues I would have them do this.
Using a one hand grip with an unloaded firearm pointed down range, squeeze the heck out of the grip till you your hand is shaking. Let off the death grip slowly till the shaking stopped. That is how hard you should be holding the firearm. Really not a tight grip at all. With the support hand hold on just as tight as with the strong hand.
Tightened up groups very quickly that way. YMMV........