Just do them frequently, that’s all it takes. Abdominals adapt very fast since they are taxed in every compound exercise. Work through it and you’ll be fine.
I also like to raise my legs to the sides instead of just straight forward to have the obliques scream for mercy. You can also do it with your legs spread at different distances for variety.
They are pretty tough. I usually do 8-10 first set, then 8-10 second set, and it goes downhill from there…7-8 third, 5-7 fourth, and about 5 the fifth.
So I do more than five sets for a total of 50 reps.
One thing that helped me take some stress off my shoulders was doing a 1/4 pullup and holding it instead of hanging “dead.”