As the Twelfth Divison Captain and leader of the Shinigami Research and Development Institute in Bleach, the Gotei 13 have certainly bestowed a great deal of faith upon Mayuri Kurotsuchi's capabilities.
This scientific mind may not have spent a great deal of energy on mastering his sword fighting techniques, however, he makes up for these substandard skills with his genius-level intellect and unashamed ingenuity. And once a greater understanding of his Zanpakuto is achieved, it becomes clear that Mayuri never needed to excel at dueling in the first place; for him, it is always a battle of the wits.
Ashisogi Jizō's Abilities
Mayuri's Zanpakuto goes by the name of Ashisogi Jizō (which translates as "The Leg-Cutting Guardian"), but this beast can do much more than sever limbs. When its Shikai form is activated as Mayuri says "Rip," this blade transforms into a warped trident with a guard shaped like a baby's head, which intermittently discharges puffs of poison into the air. Those unfortunate enough to get stabbed by one of Ashisogi Jizō's three prongs will immediately succumb to paralysis as the communication between their brain and their neural impulses are broken down by the poison damage.
The point of this Zanpakuto's contact is irrelevant; Mayuri's opponent will be incapable of using their limbs (specifically) regardless of the wound's position, and healing the laceration will not negate their immobility either. To make matters worse, the pain receptors are subsequently elevated to the max, leaving the unfortunate recipient fully aware of every agonizing injury (including the paralyzes parts.) Being the sadist that he is, Mayuri often uses this nifty feature to torture his victims (unnecessarily, although, in his mind, it is all for the sake of science.)
Mayuri is apparently not above inflicting pain onto his Zanpakuto Spirit either, as his special Ashisogi Jizō, Kyōdo Yon attack involves puncturing the eyeball of the baby on his blade. Naturally, the injured infant lets out a blood-curdling scream and begins to secrete a dark fluid from the mangled eyeball, and anyone within earshot (except Mayuri's favorites) will be incapacitated after four seconds of overwhelming soundwaves. Unlike receiving a stab wound from Ashisogi Jizō, this level of paralysis will render the entire body useless.
Konjiki Ashisogi Jizō
As with most Bankai, Ashisogi Jizō's final form is certainly a game changer, but it is safe to say that Mayuri's Zanpukto provides some of the freakiest fight scenes in Bleach. The blade's baby head is apparently hiding a monstrosity within its mouth, and when this insectile Bankai comes out to play, it's end game for most. This colossal caterpillar will follow Mayuri's every demand, even if it is still in its juvenile state. Holding firm to the theme, Konjiki Ashisogi Jizō emits a poisonous gas that has a significantly vast reach, leaving little means of escape. The poor victims do not even need to inhale the vapors to become infected, they simply need to be in the nearby vicinity.
As a man of Science, Mayuri is fully aware that one might develop an immunity to his poison attacks or formulate an antidote of their own, and so he ensures to change up the compositions of his toxins from time to time. However, this is a Shonen after all and Bleach fans love their sword fights, so Konjiki Ashisogi Jizō happily obliges with a multitude of weapons (which can each be carried in one of its numerous new appendages.) Those unfortunate enough to find themselves within the belly of the beast will not share Pinocchio's happy ending, as this Very Hungry Caterpillar has no intention of returning its snacks to the land of the living.
Mayuri's Bizarre Body Modifications
One of the most intriguing things about Mayuri is how lethal he can be without the use of his Zanpuakuto. This mad scientist has no limits to his experimentations and has even made numerous alterations to his own body, turning himself into a potent weapon that is bordering the line of immortality. Starting from the top, the 12 Division Captain has a dagger concealed within the nodes of his head and a small scythe can be pulled from his ear, which remains connected thanks to a convenient chord. Mayuri's arm can also be disconnected from his frame, leaving an elbow-lengthed stub with the weirdest-looking grappling hook attached to the end (which he can use to grab his foe from a considerable distance.)
This highly intelligent researcher has also acquired the power of camouflage, rendering himself practically invisible at the most opportune moments. Mayuri also has a backup plan on the off chance he does get injured, and ensures to keep replicas of his vital organs ready-in-waiting for an emergency transplant; how's that for prepared! He has also created a serum called Hojiku-Zai to rejuvenate missing appendages in case that is ever an issue, and as a last resort, Mayuri can always just turn into a green blob. If this Gotei 13 Captain believes his defeat is inevitable, a self-inflicted stab to the neck will transform him into an invulnerable oozing liquid, which will automatically begin making its way back to the lab and slowly remodel itself into the formidable Shinigami once again.
(More) Chemical Warefare
Mayuri can incapacitate the enemy with his own concoctions and therefore does not always need to rely on Ashisogi Jizō to get the job done. As part of his "research," this Twelfth Division Captain has created his own set of mind-bending drugs which are sometimes quite psychedelic in nature, but far from enjoyable. Whoever receives a dose of Mayuri's medication will have to navigate through a warped reality that is guaranteed to boggle the mind, rendering them practically useless on the battlefield.
The evil genius also has a Superhuman Drug in his arsenal, that can push a person's body past the breaking point, from which there is no return. The Espada, Szaelloporo is an unlucky recipient of this alternative treatment after he battled Mayuri for "The Best Mad Scientist" title and ended up with a fate worse than death. Similarly to how Marvel's Ant-Man can get lost within the depths of the Quantum Realm, Szaelloporo's mind sped up so significantly that seconds of reality felt like an eternity for the Espada, leaving him incapable to react in time to stop Mayuri from impaling him.
The Thousand-Year Blood War
The Shinigami Research and Development Institute is the first to realize that something is awry with the Hollow population in the Thousand Year Blood War, leaving Mayuri with a great deal of responsibility on his shoulders. There may still be hope, as the most intelligent member of the Gotei 13 will surely find a solution, although how many innocents will be sacrificed in the process is another question entirely. Without giving away any juicy details, let's just say that viewers should keep a close eye on Mayuri in the Thousand-Year Blood War as he pulls even more unexpected Shinigami shinanigans out of his headpiece. It seems that, even after all this time, fans have only begun to scratch the surface of Mayuri's magnificent capabilities.