If you want deep inset abs you need to train with weights period.
Toe touches
10lbs, 30 reps 3 sets
form: lift legs straight up in air while laying down, get 10lb weight and crunch up and touch the weight to your toes, exhale on the crunch and suck in gut.
superset w/
Swiss ball weighted crunch
10lbs, 20 reps 3 sets
form: lay on swiss ball, all the way down. Hold plate on forehead or behind neck if it gets annoying. Crunch up but make sure you are looking behind you/through your eyebrows througout whole motion. This ensures you will not yank on your neck/cheat on the reps and better isolate the abdominals.
Russian Twist
25lbs, 30reps 3 sets
form: sit in a "v" formation. Place weight on chest, twist left and right quickly and all the way, the momentum and balance to swing and stop the motion to go the other direction will hammer your obliques.
If you want to just tighten your core you need to do higher reps w/out weight
ex exercises:
Leg lift, bicycle crunch, 6 inches, Scissors (from 6"), swiss ball crunch, oblique crunch
reps in 30-40 range if you can, 3 sets