I've tested a bunch, and it seems like the faster you kill them, the faster they come. I think the game aims for a certain amount of enemies in the arena, and spawns more if the number isn't reached. For example, if you don't kill the first four, nothing will spawn. Only when you have a kill count of 3 will the game have 5 enemies in the arena. At 4 kills, it'll aim for 6 enemies, and so on. Which means killing them as quickly as possible after spawn is the way to go in default%, with pistol if far away. In any% it might be more efficient to save dragon slayer for more enemies.