ACHIEVEMENTS ARE NOT UNLOCKING PLEASE HELP!!!

You can do it solo. I did it solo with no problems at all. First off the Bandana Skull (infinite ammo) helps so much with pretty much every achievement, so get that if you haven't already. However this is not needed at all to get the achievement. So
all you need to do is every time you get a checkpoint, save and quit. Then reload your game, and repeat at EVERY checkpoint. If you do happen to die, simply hit the guide button and return to dashboard. Load the game again and repeat if necessary. If you
die... DO NOT hit start then revert to checkpoint. That voids the achievement. I died one time in the middle of the level on my solo run and did the technique I described above, and got the achievement. You got this!

No change, followed all instructions. I still can see all my achievements

Hey there InfamousVisionz

It seems like your account's gamerscore was reset due to cheating.

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A gamerscore correction occurs on a player’s LIVE account when they gain achievements through avoiding game play and the use of external tools. LIVE is committed to keeping the service free of this type of behavior as it violates the
LIVE Terms of Use. When abnormal achievement or gamerscore activity is found, LIVE modifies the gamerscore of players who meet specific and automated criteria.

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A LIVE player who obtained their gamerscore in this abnormal way will

• Find their gamerscore reset to zero, and
• Be unable to regain all previously earned achievements, and
• Be labeled as a “cheater” for the community to view (for members over 18 years old).
  

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While players who have been identified by these criteria will start from a zero gamerscore, they will still be able to gain achievements for future games. In other words, we give them another chance to earn future achievements by earning them as other “fair”
players do. The original gamerscore correction will remain permanent without any way to appeal.

Entire levels must be completed in one sitting for it to count towards the achievement. If you quit and save in the middle of a level and finish it later, it will not count.

I was doing some research on the game's achievements and it seems that if you used any cheat codes in the campaign they can disable the achievements. If this is the case you should replay the levels to re-earn the achievements.