The best ab strengtheners are usually done with feet on the floor. Standing ab downs in the last pull down, heavy one arm farmers walks, box squats (if done properly), various deadlifts and good mornings. Standing ab downs I was able to do 130 for 3x20 when I was at my strongest at about 235lbs bodyweight. That exercise always seemed to work for me and I could do it for months at a time pushing myself for more reps, weight, or shorter rest periods between sets. If the volume is increasing at the correct intensity your getting your training done in the same amount of time you’re going to have to work capacity to get stronger