Try this 2 exercises to tone, strengthen your abs. With proper diet and self discipline you may even get 6 packs with just this 2 exercises. These exercises works on lower abs and obliques.
Caution: Do not arch your back, it could cause back pain/injury and secondly keep your knees bend ( not straight legs ) through out your exercise to target your lower abs and prevent exerting your muscles around pelvis. Namaste