I wrote up these rules about two years ago but for whatever reason I never posted them. They're untested but to my knowledge at least decently balanced(ie you can't just have a guy flying around shooting the whole game and win like some hero encounter games).
40k Arena Battle
Rules: Choose a Body, Mask and up to 2 choices from the both the CC or Ranged section(unless otherwise stated by choices). Models play on a 4'x4' board with no terrain. Roll 2d6 to determine order of play. Phases follow the normal 40k rules.
For every round you are not in CC -1
For every hit your model does in the shooting phase or assault phase +1
For every wound your model does in the shooting phase or assault phase +1
For every finishing wound done to another model +D3
For being the last model standing +3
Body: The woman WS3 BS4 S3 T3 W2 I3 A1 Sv5+
The man WS4 BS3 S3 T3 W2 I3 A1 Sv5+
The beast WS3 BS2 S4 T4 W2 I2 A2 Sv5+
The bird WS2 BS4 S3 T3 W2 I4 A1 Sv5+ Jump Infantry
The construct WS2 BS4 S3 T4 W2 I2 A1 Sv4+
The demon WS4 BS2 S3 T3 W2 I4 A2 Sv5+ Hit and Run
Jester-It Will not Die
Sword: Parry(Force opponent to reroll one to hit roll per combat)
Plague/Poisoned Blade: Shred
Electrowhip: Opponent fights at -1WS.
Shield: +1 to Save
Spear/Trident: +1I on the first round of combat.
Shardnet: Opponent fights with one less attack
Sawblade: Counts as two options. Roll d6 at beginning of combat. 2+ You get that many extra attacks. On a 1 you make no attacks. Replaces extra attack from charging.
Two Handed weapon +1S Counts as two options
Harpoon Gun" 18" S3 Move or shoot
Spike Pistol: 12" S3
Grappling Hook: 12" S2 If hit Roll. If higher than opponent's S move them 2d6" closer. If in base combat count as having charged.
Meteor Hammer: Unaffected by Parry. 6" S3 Concussive
Razorflail: Unaffected by Parry. 6" S3. Reroll one to hit in CC.