Useful supporting work includes rotational medicine ball throws, hip hinges and squats for force, single-leg stability, anti-rotation trunk work, and shoulder endurance work. Combine it with skipping, shadowboxing, and bag work for sport-specific conditioning.
Holding a guard for twelve rounds is an endurance task. Face pulls, rows, and overhead carries build the capacity to keep the hands up, and they balance the pressing-heavy nature of the sport.
Skipping, shadowboxing with intent, bag rounds, and pad work remain the most transferable conditioning available, because they train the energy systems in the actual movement patterns. Supporting exercises should never displace them.
None of this replaces technical coaching and sparring. Two supporting sessions a week is a sensible allocation for most amateur boxers. Browse the exercise library for demonstrations and alternatives.