If you aren't feeling it, use at least a 25lb plate or dumbbell.
When you do hanging leg raise or roman chair leg raises, hold a 15+lb medicine ball or dumbbell between your legs.
You WILL feel it after this workout:
3 sets of medicine ball roman chair leg raises;
3 sets of lying leg raises (with dumbbells or person pushing down);
3 sets of side oblique crunches (use ~10 lb weight);
3 sets of Russian twists (or windshield wipers);
3 sets of weighted crunches;
3 sets of decline crunches;
3 sets of ab vacuums.
Take two days of rest (at least) after this workout. And, make sure you work your lower back out gud since it is the abdominal antagonist muscles.
-Rip