Not hard at all. Got both Sniper and Special Forces on my first try. Follow the advice from Cnote824 and use Battle Cry, and try to tap each enemy individually. Eventually, you'll get lines of enemies, you can leave your finger pressed as you won't probably miss too much. The real trick comes when you see the little ship that flies from left to right, really slowly. You can just concentrate on this ship, while most enemies will just fly into your shots. If you need to turn away do so only if there's going to be a direct hit on your turrets or shields. You can use overdrive if you get in trouble too. But if you stay on this little ship the whole time, you're gonna score 100% accuracy every time you hit it, which when balanced out with your missed shots, will yield a much higher percentage than what's needed for this achievement. The boss battle on Wave 5 also works for this. Just stay on the boss and every shot that hits is going to rise your percentage. After you finish Wave 7, just get yourself killed and both achievements should pop.