Seeking Generic Word: Place where opponents fight

Solution 1:

You appear to be looking for an arena.

a place or scene of activity, debate, or conflict.
_{Merriam Webster}

Solution 2:

Battleground is defined by Merriam Webster as:

• a place where a battle is fought
• an area of conflict or disagreement

This seems to meet the criteria for combatants only, and gives a defined boundary to the area.

Solution 3:

Field of Battle is not a single word, but Battlefield is and does fulfill most of your requirements. It is often used to describe matches between opponents other than just of actual battles;

Solution 4:

Ring is used to refer to a place where opponents fight, both literally and figuratively:

• the field of competition or rivalry

Collins Dictionary

Solution 5:

A venue simply is

ven·ue ˈvenˌyo͞o/Submit noun the place where something happens, especially an organized event such as a concert, conference, or sports event. "the river could soon be the venue for a powerboat world championship event"

It could be a boxing or wrestling match, a drag race, a ball game, or a concert. Even a Donald Trump rally. But now we're back to fighting.