From the Super Mario Wiki
Gunyolk is a large mechanical monster created by the Factory Chief, one of the most important members of the Smithy Gang, and appears in Super Mario RPG: Legend of the Seven Stars. The mechanical creature is used as a weapon in an attempt by the Factory Chief to defeat Mario and his allies. In fact, Gunyolk and the Factory Chief are the last enemies that are standing between Mario's group and the leader of the Smithy Gang, Smithy.
Gunyolk is by far the greater threat; every 2nd turn, it will unleash the deadly Breaker Beam, previously used by the Axem Rangers; unlike them, however, Gunyolk does not need to waste turns recharging after using it. Breaker Beam will easily do close to or over 100 damage to the entire party. On odd-numbered turns, Gunyolk will choose from a pair of attack pools. The first has a 33% chance of using Echofinder, which hits one character for 1.5x the damage of its normal attack and inflicts Silence, and will use a normal attack otherwise; the second pool has a 66% chance of using the Fire-attribute Mega Drain to inflict moderate magic damage, and a 33% chance of using the Thunder-attribute Electroshock, which can inflict even more magic damage.
Unusually for a boss, especially for one at the end of the game, Gunyolk has no resistance to status ailments whatsoever. If Princess Toadstool is in the active party, she can use Mute to inflict Silence on the machine, eliminating its more dangerous attacks, including Breaker Beam. She can also use Sleepy Time to put it to sleep, rendering it incapable of any action until it wakes up; if this is done, magic should be the party's primary source of damage, as physical attacks will awaken a sleeping target. If Bowser is in the active party, he can use Terrorize to inflict Fear on the Gunyolk and Factory Chief, reducing the damage they inflict and increasing the damage they take. A Fright Bomb can also be used for the same effect. Aside from status ailment abuse, Gunyolk is weak to Ice and Thunder, so Mallow's Snowy and Shocker are capable of dealing massive damage.
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