If you want to earn those abs, then you have to work hard, come what may. Here are some of hard exercise for abs that you could try,
For this exercise you will have to lie down and face the ground. Make sure you use a mat underneath. You basically have to keep yourself in a fore-arm plank position. The important tip that you would have to keep in mind is to ensure that your elbows are lined up and are right beneath your shoulders. Fist your hands and you’re good to go. The forearms must be in parallel as well. You need to hold on to this position for ten to twenty seconds, that is per set. This is considered as hard exercise for abs.
Dumbbell side bend
All you have to do is stand with your feet apart with a dumbbell in your hand (right hand). Your palm must face inwards (make sure it is towards your torso). You need to keep your back as straight as possible, trigger your core and bend to one side till the waist. You should stretch yourself as far as possible. Hold on to this position for one second and slowly return. You have to be smart while picking the weight. You can focus more if you use reasonable weights during the exercise. Keep the tempo slow.
This exercise will expect you to lie on the floor with your arms placed linearly over your shoulders. All you have to do is slowly bring your knees over the hips, bend at the knee and make sure that your calf is in ninety degree with the thigh. While doing this you need to simultaneously lower the right arm over your head and straighten your left leg by moving it towards the floor. Take a pause and then return to the initial position. Repeat the same on the opposite side. Do at least fifteen alternating reps in order to finish one set.
This is similar to dead bug exercise but the only difference is that it is an upside down version of it. Begin this in a table top position with your hips over knees and shoulders over your wrists. Activate your core when you’re simultaneously lifting your right leg and left arm. Flex your foot when you are kicking back. Repeat this for one rep on the other side as well. Do not extend your elbows.
Apart from these you could also try barbell back squat as hard exercise for abs.