I strongly feel this is way too easy and you just need to get good. Not sure how old you are, but life isn't easy and complaining about something wont get you anywhere. First off, if you played the Bounty Hunt and observed what's going on, then you would've noticed that there IS a clear marking of which imp is the real ninja. After watching the Imps I noticed that there is always 1 Imp that leads the pack of all the fake imps. So whoever is in the front (when their walking) is the real ninja. I originally thought is was the one that didn't fire his weapon, but after OBSERVING, that idea turned out to be wrong. As for only damaging the real one, I used the Peashooter. I didn't use his primary (splash damage) but used his turret ability. Just find the leader and slowly burst fire on real imp. This mission is probably the easiest Bounty Hunt in the game and your calling it nearly impossible. You need to start learning to play games and not just try something, fail, give up and then go into the PVZBFN Forums and ask the developers to make it easier.
