"Serious Sam" achievement (Serious Sam Bogus Detour)

I'm trying to get the achievement for actually beating the game (The "Serious Sam" achievement), but I'm not sure about the actual requirements. Do I have to beat the game completely unmodified (game switches, not mods), or do I have to beat every level in a save file? I played on a save file onto the last level, but I screwed up and ran out of time, and had to restart the level. I quit the game for a while, and when I came back my save file had corrupted. I was not willing to restart the entire game, so I restarted the final level. I beat it, but I got no achievement. My problem is that I'm not sure if this is because I had game switches turned on, or if It's because I hadn't beaten every level on that save file I actually beat the game on. I've been looking all over the wiki, and I can't find the actual info I need.