What is a battle in the Gunman daily mission?

A battle is one of the sequences in War Zone. The ones that one fights with the autoclicker. A battle is killing one set of units. The units are numbered. So if there are fifteen units in a battle, it will say 1/15, 2/15, ..., 15/15. When you destroy the last unit (the 15th of 15 in this example), you win the battle.

The daily mission is triggered on the win. You do not have to destroy all the units in a battle to finish the daily mission, just the last one. So you do not need to start a new battle if you are already in one.