If you finish Halo 2 legendary in under 3 hours in coop, the achievement will not unlock, unless you finish Halo 2 solo legendary with any time. I just completed the game solo legendary using the scarab skull and after completing the last level I needed it gave me the achievement. I hope anyone else having the same issue reads this because this little detail has been quite elusive on the internet.
Can i use the scarab skull on coop too?