If your that concerned about squeaky voices bragging, just ruin Christmas for them, that'll shut them up for awhile.
I dont think classic will be hard actually. Halo 3 ODST is more about stealth than Halo 3 since ODSTs are weaker so you can probably avoid enemies in a lot of areas. Also wasn't one of the mission in halo 3 a cutscene? The achievement might unlock if you watch a cutscene on legendary difficulty. Also if theres a mission like cortana, you can just run through it trying to avoid everything.
Deja Vu is the most I'm worried about, I tried to get annual 3 times, but the first time my team sucked, the second time someone left early. The final time took me an hour and 45 min to do so total I think I spent like 4 hrs trying to do it.
Endure sounds easy, Im not going to worry about it.
So more ten year olds my have it then you think.
[Edited on 07.19.2009 9:07 AM PDT]