Battle ropes are great for training to build both cardio and "standing" core strength. To add single leg balance and stability to the mix, try using battle ropes on one leg.
Slightly bend the leg that is in contact with the ground and tilt your torso forward as you perform the exercise.
For an even greater challenge to you balance and stability. perform on one leg while standing on a bosu ball or other unstable surface (see the vids)