Overview:
-Estimated achievement difficulty: 8/10
-Offline: 38 (915G)
-Online: 11 (85G)
-Minimum number of playthroughs needed: 1-2
-Number of missable achievements: None can use mission select to redo missions for achievements
-Do cheat codes disable achievements?: N/A
-Does difficulty affect achievements?: Yes Minimum difficulty must be Normal some require at least Heroic or Legendary
-Glitchy achievements: none

Campaign (915)

All these achievements can be done through just playing the game in campaign mode. Some of the side achievements can only be gotten on certain levels or are really easy to get on others.

A Monument to All Your Sins 150
Completed every mission in Halo: Reach alone, on Legendary.

There is plenty of advice on how to beat the game on legendary on solo HERE some advice though. This will probably take you in the range of 8hrs-20hrs+ depending on your gaming skill. Using the plasma pistol and either the DMR or Pistol is a MUST. Headshot everything possible and use the plasma pistol to take elites shields off and the one shot their heads with pistol/DMR. Bungie did a great job of creating weapon stashes in spots where you will need them. So if you get to a hard part where you seem like you could use more fire power simply look around. Exploring areas I normally wouldn't go (even using a jetpack) earned me quite a few unknown rockets/snipers that made tricky parts a LOT easier. Take your time... don't rush the more you die the more you reset and get frustrated. Lastly, there are plenty of spots you can simply sprint past and just reach the checkpoint so look those up. Plus, spartans can't die like the marines so let them kill some enemies for you.

Gods Must Be Strong 125
Completed the Campaign on Legendary difficulty.

Same as A Monument to All Your Sins, but you can do this co-op to make it easier if you don't feel like playing solo to get it.

Folks Need Heroes... 50
Completed the Campaign on Heroic difficulty.

Same as the last just easier to do.

The Soldier We Needed You To Be 25
Completed the Campaign on Normal difficulty.

Same as the last again just even easier to do.

We're Just Getting Started 10
Completed the 2nd mission on Normal or harder.

Pretty simple finish the level on normal or harder to get this achievement. Co-op is allowed to do this. Note: this level blends in with the first level so it may seem a little long. Plus, there first level doesn't have an achievement to unlock for completing it so don't worry.

Protocol Dictates Action 10
Completed the 3rd mission on Normal or harder.

Refer to We're Just Getting Started

I Need a Weapon 10
Completed the 4th mission on Normal or harder.

Refer to We're Just Getting Started

To War 10
Completed the 5th mission on Normal or harder.

Refer to We're Just Getting Started

You Flew Pretty Good 10
Completed the 6th mission on Normal or harder.

Refer to We're Just Getting Started

Into the Howling Dark 10
Completed the 7th mission on Normal or harder.

Refer to We're Just Getting Started

Dust and Echoes 10
Completed the 8th mission on Normal or harder.

Refer to We're Just Getting Started

This is Not Your Grave... 10
Completed the 9th mission on Normal or harder.

Refer to We're Just Getting Started

Send Me Out... with a Bang 10
Completed the 10th mission on Normal or harder.

Refer to We're Just Getting Started