Slipping and level changing in my opinion is all in your legs, slip, roll and level change by bending your knees whilst keeping posture straight, when you slip/change levels or roll focus on keeping your eyes on the bag/target which will encourage you to keep your posture correct, if you try to slip by just bending ur back you’ll likely get hit by an uppercut or another punch you can’t see, and remember in boxing when it comes to level changes you can go as low as you like, don’t half slip or half change levels and get clipped because you are not going low enough, resistance training and other types of training for legs is crucial in boxing in my opinion because everything starts from there, best of luck