Dynasty Warriors: Gundam 3

This achievement is cumulative, and does not have to be done in one mission, you can check how many you have shot down in Player Info.

You will get this quite quickly as each stage you are bound to shoot down at least 200 minimum, if you go for the enemy HQ first on each mission you still have to destroy at least 100 MS to get the commander into battle anyway.

This achievement requires that you kill 10,000 enemies, although not all in the same stage.

If you intend to 100% this game, you will get this way before you even put a dent in all the missions, even if you blitz the commander as fast as possible as every time.

Most people won't need longer then 10 hours for this.