Note This is one of my first projects, and while it does work, it is NOT an easy build.
Found here are all the files for printing a one pound "antweight" combat robot. These files are specifically for the all plastic version, used in the 3d printing friendly "Plastic Ant" class our local robot fighting club has started. For that version, I use eSun PLA Pro which is perfect for this sort of thing. I also have a standard ant version with a metal blade and I used nylon instead of PLA Pro for the front legs and wheel guards (nylon is prohibited in the Plastic Ant class.) The Nylon version is basically the same, though some sort of hub for a metal weapon would need to be crafted. The nylon holds up surprisingly well to even the most dangerous ant weapons. I've yet to try PLA Pro against normal ants, but I suspect it'll be insufficient to handle direct hits.
Filament: ESun PLA Pro (NOT standard PLA), or Taulman Bridge Nylon for front legs and wheel guards.
Outside Perimeter: 5mm
For non-plastic, a blade of some sort. A sawblade will do, but there are better options.
2x 2" GWS Foam Wheels (hub removed) - http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHJA2&P=FR&gclid=CIHl2pHNsssCFRSUfgodaRcA8w
1x Brushless outrunner, 28mm diameter -http://www.hobbyking.com/hobbyking/store/__65236__D2822_14_Brushless_Outrunner_1450kv_AR_Warehouse_.html?strSearch=d2822
2x speed controllers (ESCs) for the gear motors - http://www.fingertechrobotics.com/proddetail.php?prod=ft-tinyESCv2)
1x brushless speed controller for the weapon motor - http://www.hobbyking.com/hobbyking/store/__58265__Flycolor_20_Amp_Multi_rotor_ESC_4_6S_Opto.html?strSearch=flycolor
1x 11.1v 250mah LiPo battery - http://www.motionrc.com/admiral-250mah-3s-11-1v-30c-lipo-battery/
1x Receiver - http://www.hobbyking.com/hobbyking/store/__61211__OrangeRx_R410X_DSMX_Compatible_4Ch_6CH_PWM_CPPM_2_4Ghz_Receiver_.html
2x Micro Gearmotos http://www.robotmarketplace.com/products/0-PL998.html (this is the example, but they can be found for far cheaper on ebay)
1x MXL Timing pully
1x MXL Timing belt
Lots and lots of 4-40 machine screws (asst lengths) and nuts
What happens when a bot made of PLA, Mister Twister, fights a bot made of titanium, Dark Pounder.