A Huge-size Battlesuit didn't seem like the right way to make a Gumelef or Gundam, so I used a combination of the rules for constructs and vehicles. Comments are welcome.
A mecha is a heavily armed and armored combat vehicle with a humanoid form. Mecha are usually at least Large size, and are often much bigger.
Base speed for mecha is 30 feet, modified by size and applicable powers.
Mecha use the rules for constructs to determine their Strength, Dexterity, Protection, and hardness.
Mecha have all the Immunity super-feats. Immunity to energy confers its benefit on the mecha’s occupants as well as the mecha itself.
For a cost of 4pp, a mecha can give it’s occupants Immunity to pressure and suffocation.
Mecha have no Constitution scores. They are immune to effects which require Fortitude saving throws, unless the effect works on objects.
Mecha take damage like vehicles. An injured result imposes a –1 penalty on subsequent damage saves. A stunned result shuts down either its legs (rendering unable to move), arms (making it impossible to make melee attacks), or shutting down one super-power. A successful Repair check against DC 15 brings the damaged system back into working order. A disabled result indicates total shutdown. A successful Repair check against DC 30 brings the mecha back online.
Mecha have no Intelligence, Wisdom, or Charisma scores. The are incapable of acting independently. Mecha are typically operated by a pilot from inside. If a mecha is subject to an effect that requires a Will saving throw, it is unaffected. Anyone inside the mecha is considered the target of the effect instead.
Defense for a mecha is:
10 + pilots base Defense bonus + mecha’s Dexterity bonus + mecha’s size modifier
Melee attack bonus for a mecha is:
pilot’s base attack bonus + mecha’s Strength bonus + mecha’s size modifier
Ranged attack bonus for a mecha is:
pilot’s base attack bonus + mecha’s Dexterity bonus + mecha’s size modifier
Most feats can’t be used while piloting a mecha. A pilot must take a “mecha version” of any feats he wants to use while piloting a mecha, which applies to mecha use only. The prerequisites are the same, except that Strength and Dexterity requirements refer to the mecha’s ability scores, not the pilot’s.
Mecha Feat List: Accurate Attack, Arial Combat (mecha must have Flight power), All-Out Attack, Ambidexterity, Attack Finesse, Attack Focus, Evasion, Expertise, Improved Disarm, Improved Trip, Improved Critical, Improved Grapple, Improved Initiative, Improved Pin, Instant Stand, Lightning Reflexes, Move-by Attack, Point-Blank Shot, Far Shot, Precise Shot, Rapid Shot, Multishot, Ricochet Attack, Power Attack, Takedown Attack, Rapid Takedown, Quick Draw, Rapid Strike, Stunning Attack, Surprise Strike, Throwing Mastery, Two Weapon Fighting, Improved Two Weapon Fighting, Underwater Combat, Whirlwind Attack.