Right Arm has three attacks; a physical attack, Calamity, and Mighty Guard. When Penance is not under Regen, there is an equal chance of any attack being used. When Penance is under Regen, there is a 66% chance of a physical attack, and a 33% chance of Calamity.
Right Arm's physical attack inflicts Petrify and shatters the affected character; this ignores Stone Ward but does not ignore Stoneproof or Ribbon. Calamity inflicts Poison, Full Break, Curse, Darkness and Silence on one character, also ignoring Wards but not ignoring full protection. Mighty Guard gives itself, Penance and Left Arm the status effects: Protect, Shell and Regen.
When Right Arm is defeated, it regenerates shortly afterwards (12 ticks). If killed via Doom, it takes twice as long to regenerate.
Right 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.
Right Arm does not give any additional reward for Overkill.
With adequate Strength, the Right 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.
Right 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 Right Arm's drops accumulate each time it is killed, it is possible to farm it for Dark Matter or Master Spheres.