- Spartan 104 S2AM
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- Exalted Mythic Member
"Don't sweat the petty things and don't pet the sweaty things."
"Waiting is good. It means you’re not going to die. The person you need to feel sorry for is the one who gets rushed into the ER and treated first."
NREMT-B
Tiocfaidh ár lá!
I'm not quite sure of your scenario, but I will try my best to give you an answer.
If I understand correctly, you have completed all but the last 2 levels on legendary (and have the shields to prove it. This is important, as it is a record of your completion).
If that is the case, restarting your campaign or selecting a level will not reset your completion stats. I did not complete the campaign in one play though, nor was it even in order. As long as you have completed every level at least once on legendary, you should get the achievement.
You may have some other mitigating factor I didn't notice, but from what I read, you should be pretty safe.
[Edited on 01.27.2011 1:11 AM PST]