Probably the easiest battle-ready Pokemon to obtain, it's Aron! Formerly even its final evolution stage wasn't much to gawk at metagame-wise, but in Gen V Aron can give the unprepared player quite a surprise. Behold!
Aron #304
HP: 50
Atk: 70
Def: 100
SpA: 40
SpD: 40
Spe: 30
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Level-Up Moves
-- Tackle
Lv4 Harden
Lv8 Mud-Slap
Lv11 Headbutt
Lv15 Metal Claw
Lv18 Iron Defense
Lv22 Roar
Lv25 Take Down
Lv29 Iron head
Lv32 Protect
Lv36 Metal Sound
Lv39 Iron Tail
Lv43 Autotomize
Lv46 Heavy Slam
Lv50 Double-Edge
Lv53 Metal Burst
Machine Moves
TM01 Hone Claws
TM05 Roar
TM06 Toxic
TM10 Hidden Power
TM11 Sunny Day
TM17 Protect
TM18 Rain Dance
TM21 Frustration
TM26 Earthquake
TM27 Return
TM28 Dig
TM32 Double Team
TM37 Sandstorm
TM39 Rock Tomb
TM40 Aerial Ace
TM42 Facade
TM44 Rest
TM45 Attract
TM48 Round
TM65 Shadow Claw
TM69 Rock Polish
TM78 Bulldoze
TM80 Rock Slide
TM87 Swagger
TM90 Substitute
TM94 Rock Smash
HM01 Cut
HM04 Strength
Egg Moves
Body Slam
Curse
Dragon Rush
Endeavor
Head Smash
Iron Head
Screech
SmellingSalt
Stealth Rock
Stomp
Superpower
Gen IV Tutor Moves
(Pt/HG/SS) AncientPower
(Pt/HG/SS) Earth Power
(Pt/HG/SS) Endeavor
(Pt/HG/SS) Fury Cutter
(Pt/HG/SS) Iron Defense
(Pt/HG/SS) Iron Head
(Pt/HG/SS) Magnet Rise
(Pt/HG/SS) Mud-Slap
(Pt/HG/SS) Rollout
(Pt/HG/SS) Snore
(Pt/HG/SS) Spite
(Pt/HG/SS) Superpower
(Pt/HG/SS) Uproar
(HG/SS) Headbutt
Aron -> Double Battles
->Endeavor<-
Aron @ Shell Bell / Berry Juice
SASSY (But whatever, really)
+Sturdy+
(0 HP IV)
-Endeavor
-Sandstorm
-Protect
-Toxic
As simple as it gets: have sandstorm up, get hit, go down to 1 HP, Endeavor the foe down to 1 HP, drink up its health back to full HP, and watch sandstorm whittle away that one HP on the opponent.
This set could theoretically last forever, but there are plenty of problems. The existence of Ghost-types, Mold Breaker Pokemon, health-sapping statuses, and multi-hit moves are the main stipulations in Singles; in Doubles, the knowing opponent can set both his Pokemon on Aron and KO it before it gets to Endeavor. That's why Berry Juice is an option: it guarantees that Aron will survive two ordinary, straightforward attacks.
Sandstorm and Toxic are almost fillers, because 95% of the time you are going to want to either Endeavor or Protect. In a 1-on-1 scenario, you have to have SOMEthing to hurt the foe besides Endeavor if you want to win. I'd just as soon leave the last slot blank so I can't misclick it, if that gives you an idea of how rarely I would actually use Toxic.