Daniel

Your level stats remain the same throughout, as does collecting orbs.

If your friend hosts you join his single player game, and although orbs/skills you get save - the bosses being killed will not - if you start and he had already located/killed at least 1 boss - and you joined after you would have only killed 6 bosses. The game remembers who you have and have'nt killed wether its your game or not.

When i got the achievement i had already completed the game, and my friend halfway through - we had to do the bosses in Time-trial mode to unlock the complete all missions one