For details see STILL SOMETHING TO PROVE (it's the same, expect for the bugs).
Realism Mode on expert is very challenging. The main differences between normal and realism are:
- In Realism there are no outlines around the players. However, there are around the gas cans when you have to collect them!
- There are no Revival Rooms, so always keep a defib.
- No outlines around items and weapons, only when you get very close to them.
- You have to go for the head all the time, because the weapons are underpowered, most notably snipers (it takes 3 hit to the body to kill 1 infected, without Realism it takes 1 shot). M60 and Magnum are the only 1 hit weapons (plus the melees)
You can get this achievement on ANY campaign levels, not just on the original 5 for L4D2 (Center, Carnival, Swamp, Rain and Parish).