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There is are different strategies you can use for each challenge. 1. You need 100% accuracy and complete under 60 seconds. Practice sniping. Take out the hardest targets first for each run. 2. Unlock the GLOCK 17 (50 pistol kills) and learn the sequence. You don't need a perfect run, but close to it. 3. You just need 100% accuracy. So hit someone with one bullet and then die. Easy. 4. Use the XM1041 shotgun (complete game on novice). Should be easy after that. 5. I recommend the M4A1 on semi-auto. Find a firing sequence for each stage that feels comfortable. For the last stage, hit alternating targets so you're not waiting for the target to come back up
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I have the illusion that people aim better as soon as they got a headshot. More often than not I get a "counter shot" right after I should have splatted their brain.