Left 4 Dead had the achievement What Are You Trying to Prove? earned by completing all the campaigns on expert.
L4D2 had Still something to prove, again by beating all the campaigns on expert.
I was able to get the second one by playing Last Man On Earth, which is single-player, but special infected only. The higher difficulties are mostly harder by having common infected do more damage.
Left 4 Dead had the achievement What Are You Trying to Prove? earned by completing all the campaigns on expert.
L4D2 had Still something to prove, again by beating all the campaigns on expert.
I was able to get the second one by playing Last Man On Earth, which is single-player, but special infected only. The higher difficulties are mostly harder by having common infected do more damage.