i need help with halo 3 i cant get acheivment?

[asker13]

ok so on halo 3 i just got it a couple days ago and i ve beat it before but i got a 360 and i started at the beggining so i was trying to get the first skull and i was on legendary and it wont unlock my acheivment why??
i started straight on legendary
i got a new 360 and halo3 a couple days ago so and playing on legendary because i played alot before with friends but im trying to get the skull and it work...

 

[Toby S]

if you let me know exacly what skull it is i could tell you. and where the skull is.