How do I make a battle end only by killing a specific enemy while other enemies remain alive?
Others really stay alive
Simply set the "Win battle even if alive" bitset on all enemies except the one that needs to be kiled. The enemies will remain on the battlefield at the end of battle.
You can alternatively set the "Boss" bitset on the enemy you want to require to die. Then go to the other enemies' bitsets and select "Die without Boss". They'll die when the boss does. You can also set "Flee instead of die" so that they'll run instead.