Here's an utterly cheesy recipe for a level 15 killing machine:
Version 1:
Take one kobold. Make it a 1st level rogue.
Add 6 levels of sorcerer and 5 levels of Spellwarp Sniper (Comp. Sc.), and just a pinch (1 level) of Assassin.
Finish with a nice coating (2 levels) of Arcane Trickster.
The finished product should have AC through the roof and some
very lethal ranged attacks. It will probably have a misearable Strength score, but since you'll be making all your attacks with rays who cares?
Version 2:
Same as above, but instead go halfling rogue 1/wizard 5/spellwarp sniper 5/assassin 1/arcane trickster 3.
Version 2 is obviously the most effective, since the high Int of a wizard will help you get the many skill points you need to qualify for the various prestige classes, and the one extra level of arcane trickster earns you an additional +1d6 sneak attack. But since "anything goes", this could be your big chance to play a kobold PC!
Of course, you could always start with a kobold rog/wiz, but that would give you an XP-penalty. Either way, you should have a character that simply oozes wrong, bad fun.
PS: Points to consider: A kobold has +1 natural armor, but a halfling has +1 racial bonus to all saves. Compared to kobold, a halfling has +2 Str and +2 Con, so I guess halfling is the true munchkin way to go. But remember, you could be playing a
kobold!