A Lemz0r is a holy (and mostly untouchable) master of Lemmings. To achieve Lemz0r status one must either have completed an entire version of Lemmings, or whence given the challenge sit down and beat any level on demand.
This definition primarily pertains to all incarnations of TEAM17's game Lemmings dating back to the Amiga, but can also be used to describe a tamer of literal Lemmings if necessary.
Also a Lemz0r is normally an above average Worms player, or even perhaps a Wormz0r.
Friend 1: "I'm stuck on this psychotically difficult level in Lemmings, it's fucking impossible."
Friend 2: "I'm sure it's not impossible, hold on I'll call over Max, he's a Lemz0r, he can figure it out."