Left Arm has three attacks; a physical attack, Slowga, and Mega-Graviton. If there is at least one character not under Slow, there is an equal chance of all three attacks. Otherwise, there is a 66% chance of a physical attack and a 33% chance of Mega-Graviton. In the International version of the game, Left Arm's Mega-Graviton was named Tera-Graviton; this was changed to Mega-Graviton in the PAL version, which was carried over to the HD remaster.
Left Arm's physical attack removes several beneficial status effects: Auto-Life, Haste, NulAll, Protect, Reflect, Regen and Shell. Slowga simply inflicts Slow on the whole party; like any Slowga, it can be reflected. Mega-Graviton inflicts damage equal to 75% of the characters' max HP, and additionally inflicts Doom, Slow, Sleep (by 99 turns), Darkness (by 99 turns) and Silence (by 99 turns).
When Left Arm is defeated, it regenerates shortly afterwards (12 ticks). If killed via Doom, it takes twice as long to regenerate.
Left Arm's Agility is 150 when it first spawns or regenerates, but is raised to 255 once it gets a turn. Additionally, it effectively has a "stealth Haste" effect; in that it gains Haste status before each turn and loses it immediately afterwards, but keeping the benefit of the quicker next turn. This creates the illusion that its attacks have a Quick Hit property.
Left Arm does not give any additional reward for overkill.
With adequate Strength, the Left Arm can be defeated in six Quick Hits. By finishing it off at the last opportunity before its turn, its regeneration can be delayed, giving the party the most opportunity to attack the body.
Left Arm is very evasive, to the point that no amount of Accuracy will have any impact on the chance of hitting it. In order to reliably connect, the characters' Luck must be raised. At 85 Luck, hitting it is guaranteed unless the attacker is under Darkness; at 100 Luck, it is guaranteed even under Darkness.
Since it is possible to escape from Penance, and Left Arm's drops accumulate each time it is killed, it is possible to farm it for Dark Matter or Master Spheres.