Originally posted by another user
Modern Warfare 2 introduced a new form of customization called Callsigns. These callsigns are varied and unique allowing players to distinguish themselves amongst an ocean of players. Most of these callsigns are unlocked by complete specific tasks or by perform certain actions.
Out of the 592 possible titles here are WRIXEL’s the TOP 10 Hardest to Achieve Callsigns in Modern Warfare 2:
Challenge: “Droppin’ Crates” - Get a Game Winning Killcam by dropping a crate on the enemy.
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Challenge: “All Pro” - Get 2 headshots with 1 bullet.
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Challenge: “Invincible” - Fully recover from enemy damage 5 times without dying. Screen must turn red/bloody
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Challenge “End Game: II” - Call in 5 Nukes.
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Challenge: “How the?” - Kill an enemy by using bullet penetration to blow up an explosive through a wall. FMJ is not Required
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Challenge: “Think Fast Flash” - Finish an enemy by hitting them with a flashbang (Direct impact).
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Challenge: “Perfectionist” - Fire an entire Sniper magazine into your enemies without missing.
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Challenge: “Last Resort” - Get a game winning Killcam while in either Last Stand or Final Stand.
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Challenge: “Stickman” - Stick a Semtex to the enemy in a Game Winning Killcam.
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Challenge: “The Brink” - Get a 3 or more kill streak while near death (screen flashing red).
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I didn't make this, personally I'd add the last Spectre challenge in there, maybe a few other Killstreak Prestige challenges.