On the second map, there is a room near the end with x2 damage and fast shot power-ups. Just run to the end, bypassing the enemies, shoot into the room to pop the plant things, pick up the power-ups and run back out to kill 3 enemies really quickly. I'd avoid the special enemy since he takes more shots to kill and just go for the regular guys. They fall right over with the power-ups.
You can also kite a few enemies over before grabbing the pick-ups to make it easier, but I didn't do this and still made it under the 10 second bar. If you blow it, just restart the level. Running to the end takes less than a minute.