The Beans wrote:You can definitely unlock the Achievements if you use a save but to unlock the stuff like the "Landfall" Achievement you've got to do it on the same difficulty. So you need to have completed the first three missions on at least Normal.
Maybe they only unlock if you do all missions on the same difficulty.
I definitely saved and quit mid-level but never changed the difficulty at all on two playthroughs.
I would say that dropping to Easy and then restarting mid-level would stop the Achievement unlocking. Otherwise you could just keep dropping the difficulty on the mad bits but still unlock the Achievement for completing the game on Legendary etc.
Also, Halo 3 performs worse if you install it to HD according to Bungie. Unless that's been patched. It's been a while since I gave it a good play. I installed mine anyway and didn't notice anything tbh.
I'm pretty sure that dropping to easy for a section, then going into the next section on a harder difficulty would wreck things, but when I resorted to scouting a section on easy or normal, I replayed that section again on Heroic.
It seems that you have to continue a play though in one go - you can save, then use resume to continue, but it seems dropping out of a game, then using the Campaign menu to restart from a specific point breaks things.
Whether this has any bearing on not gaining any achievements I don't know. I'm mid-way through Sierra 117 on Normal at present (and it's dull and no challenge at all).
I found the first skull, but it's a silver skull which doesn't give achievements (for anybody), so I still don't know if this is going to work until I find the first gold skull on this level, or just complete the level.
The achievements in this game are far less clear than on everything else I've played...