Thanks to dethklok for starting the original thread but it's not getting updated as fast as it needs to be.
Overview
+ Estimated achievement difficulty: ?/10
+ Offline: 50 (1000)
+ Online: 0(0)
+ Approximate time: ?
+ Minimum number of playthroughs needed: ?
+ Glitched achievements: None
+ Cheats: No Cheats
Co-Op does not affect achievements, anything that you can unlock in single player will work exactly the same in Co-Op.
Infinite ammo does not affect achievements.
Beat the game in under 5 hours (any difficulty) to unlock Infinite Rocket Launcher.
Story Related - Unmissable
Completed Chapter 1-1 - 15
Complete Chapter 1 - 1 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 1-2 - 15
Complete Chapter 1 - 2 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 2-1 - 15
Complete Chapter 2 - 1 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 2-2 - 15
Complete Chapter 2 - 2 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 2-3 - 15
Complete Chapter 2 - 3 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 3-1 - 15
Complete Chapter 3 - 1 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 3-2 - 15
Complete Chapter 3 - 2 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 3-3 - 15
Complete Chapter 3 - 3 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 4-1 - 15
Complete Chapter 4 - 1 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 4-2 - 15
Complete Chapter 4 - 2 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 5-1 - 15
Complete Chapter 5 - 1 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 5-2 - 15
Complete Chapter 5 - 2 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 5-3 - 15
Complete Chapter 5 - 3 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 6-1 - 15
Complete Chapter 6 - 1 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 6-2 - 15
Complete Chapter 6 - 2 on any difficulty setting.
See "Completed Chapter 6-3"
Completed Chapter 6-3 - 15
Complete Chapter 6 - 3 on any difficulty setting.
Complete the chapter on any difficulty and this achievement will unlock when the game breaks down your stats for the level and gives you your rank.
Recruit - 15
Complete all chapters on Amateur.
See "Veteran"
Soldier - 15
Complete all chapters on Normal.
See "Veteran"
Veteran - 30
Complete all chapters on Veteran.
Complete the game on a chosen difficulty to unlock the appropriate achievement. Difficulties do stack so you can dive straight into Veteran if you're feeling brave.
War Hero - 70
Complete all chapters on the most difficult setting.
Complete the game on Professional difficulty. To unlock this difficulty you first need to complete the game on Veteran.