Some of the posts here are totally wrong. The majority of achievements can be done across multiple playthroughs. The only ones I know for sure you can't are the gifts for Katey, the zombie genocide, escorting 50 survivors and spraying the Zombrex posters. I believe everything else carries over.
I'm about ready to start beating my head with my keyboard. I need the second Monster Mash cliche and I just can't figure it out. I even googled lists of RPG monster cliches and D&D monster lists. I'll probably feel stupid for not guessing it sooner, but BYARGHLE I'm going crazy.