I'm googled it and cannot find an answer. I'm looking for a single word to describe multi opponent combat or multiple opponent training. Maybe even in the way of a skill. For example, a single person fighting multiple attackers at once. A single word....
a confused hand-to-hand fight or struggle among several people.
A Battle royal is a fight involving three or more combatants, last fighter remains standing. This can include factions or be a free for all.
American Heritage Dictionary
This may not satisfy your one word request but it fits well into the description required.
A One Man Army is a heavily armed and well-trained soldier... Usually he has enough muscles, brains, tactics and daring to face and defeat countless enemies by himself, hence the term: a single soldier who is worth an entire army due to his extreme combat competence.
Hence multi opponent combat.