A little more explanation is needed I think. To complete this challenge, you have to:
1) Kill 10 plants (purple in the middle, look like a star)
2) Kill 25 birds
3) Fly through 10 smoke rings
and to get the cheevo, you need to do it in less than 2 min 50 sec. It took me about 4 tries before I got this.
What worked for me was to just keep the fire button depressed all the time...it will shoot rapid fire and probably faster than if you tried yourself.
Next, if you see one of the plants to shoot, do so immediately...only 10 of them are needed, but I found that when the time is ticking down, you'll forget where they are and struggle to find them. So, shoot them when you see them.
Finally, as you fly around, try to fly through as many rings as possible as early as possible while continuing to shoot...doing two or even three things at once will net you this achievement.