These multi-planar movements fire up your core, while punching a heavy bag helps develop serious upper-body strength. Since most boxing classes also incorporate bodyweight exercises, such as squats, lunges, and push-ups, you're essentially checking all the boxes on the list of "must-do's" when it comes to leaning out fast.
The "Big 4" compound exercises
If I could only choose four exercises to prescribe to anyone seeking to shape up in a hurry, I'd point to squats, lunges, pull-ups, and push-ups. The reasoning is simple: These exercises can be modified for practically any fitness level, they can be done with or without added resistance, they target all the major muscle groups, and they're reasonably accessible with or without a gym membership. Plus, there are about a million variations you can incorporate to help keep your workouts interesting.