Halo: Combat Evolved Anniversary

For this achievement you have to do the level "Halo" without ever entering the warthog this includes riding shotgun and gunner. It's a little time consuming to walk the whole level but it's pretty easy on easy or normal difficulty. This achievement can also be easily done with a co-op partner simply let them drive the warthog and just let them warp you around the level. If your doing this one solo though you're going to have a little problem near the beginning. After the first battle with the marines by your side a warthog gets dropped and you have to drive through a cave. There is a point where you have to jump the warthog to get across because it is to far to just jump. But there's away around this using the grenade jump all you have to do is follow the video below to help you with this one. Remember to have a couple extra frag grenades for this because the plasma grenade is just to frustrating. It took me a couple tries to get this one just reload checkpoint if you mess up. The achievement will pop once you complete the level after the cut-scene. I take no credit for the video thank Majateka for this one.

Simply make it through the second level in the campaign "Halo" without ever entering the warthog. So basically you just have to haul ass and make you're own way through the cave and rescue the marines without it. Fairly straightforward, just time consuming, the only problem is the jump that is in the first cave that cortana informs you is not a natural formation, where you will have to grenade jump over the gap so make sure you don't use all your nades up in the prior fighting (plasma greandes Will work, probably better as they dont bounce so are easier to place where you want them) It can be a little frustrating to jump but is very possible if you get it right (FYI the terminal for this level is also located at the bottom of this ramp, just round the corner)

Edit: It is also possible to do this in co-op if the other player drives, jumps the ramp then teleports you, the player that drives however will obviously not get the achievement themselves though.
Credit to Case Dang It

The easy way to get this achievement is in Co op. You'll get to a point in the tunnel where you will not be able to cross. The solution is to have one partner in the warthog and jump the cap with the warthog. Eventually the other guy on foot will re-spawn next to the warthog and you will be able to continue on foot. The end of the mission will come and "Bing" achievement will pop up!