A V-sit (or boat pose for yogis) is an excellent core exercise, but it primarily targets your abdominal wall. By adding a rocking motion you’ll also engage your obliques.
To do it: Lie faceup on the floor with your legs and arms straight. Cross your left leg over your right. Lift your torso and legs so that your body forms a wide V. Gently rock your body to the right, lifting your left shoulder and hip off the floor, bracing your core to stop your body from tipping over. Rock back to your left and lift your right shoulder and hip. That’s one rep. Do 3 sets of 10 reps.