Are Halo MCC Achievements based on personal performance?

Question

With the campaigns of the available games done, my friend and I plan to tackle some of the extra achievements.

I.e. "Look Out for the Little Guys", which requires to complete Two Betrayals on Legendary without killing any Grunts. If we jump into coop and he kills all grunts while I concentrate on other enemies, will I be getting this achievement? Similar to "Leave It Where It Lay", where on Two Betrayals on Legendary, we are not allowed to pick up any new weapons. If I pick up any weapons while he keeps the starting weapons, will he be getting the achievement?

So can one player in the session work on an achievement while the other player "ignores" it?

Answer 1

Yes, those achievements can be achieved by you, if you play the level in co-op and your friend does not play the level according to the achievement criteria.

I have tested this with my brother lately and it is also stated here. There, it also says that you could use local co-op (if you have a second controller) and use Player 2 to play the level normally and get the achievement for Player 1 (actually you), who would just stand around idle.