Bannai

Introduction
Bannai is a Reaper Demoman appearing in Episode 6, in previous versions of this stage, he was seen fighting alongside the Reaper Pyro, which was later removed and appeared in Episode 13 at the end, Metalgen, known at this point as Robot Spy, is seen at the top area within a Control Room, talking to the team about something and then the Robot Heavy says he'll kill everyone, specifically Pootis Mann (referring to the Heavy), but Robot Spy is quick to say no to that, wanting to save him for later, saying he'll only send Bannai to fight them, he doesn't mention the name of the other 2 Reaper Robots.

Stages(excluding health and damage dealt)
Bannai currently has 4 stages

In stage 1, he is seen using the bottle to fight, which does quite a lot of damage, but he doesn't regenerate any health on kill.

In stage 2, he becomes a Demoknight, sporting the Eyelander and the Chargin' targe, no boots are seen, but he has 100% turning control during this, as most Demonknight Robots do

In stage 3, he has the Half-Zatoichi, it is recommended not to kill him with the same weapon since it bugs the script, same goes for any boss and lightning spells, once he kills you, he restores ALL health he lost, if he kills you when he's near full health, he'll recieve overheal, but not above 200%, so, if he had 200 HP normally, he wouldn't go above 400 hp with Overheal

In stage 4, he uses the Pipe Bomb Launcher, kind of like the Beggars Bazooka but extremely over exaggerated, if he looks at you and attacks, his pills go everywhere until his clip is empty, he can't crit with this weapon and it has the normal blast radius as far as I've seen

Surprise!
After you beat his 4th stage, he'll have a surprise for you, a BOMB! Once he says that, and the script says that you need to get out, destroy the door where the sign says so, you have 5 minutes to escape before the bomb explodes and takes the facility, and you, with it, if everyone dies, you have to restart the mission, which is what Bannai is going for here.

Strategies
Bannai is ruthless, but dumb, if he sees a Sentry, he'll go for it all the time, so, have a few engies before this, because his melee can damage the Sentry a lot, once 1 Sentry is down, he'll go for the next, giving the engie who lost his sentry the opportunity to build another, Pyro is kind of useless during this battle, aside from cancelling his charge, Heavy is useful here if you can aim well, Spy and Sniper are horrible options in this battle because he keeps on moving in his second and third stage, and 4th stage is a pain for Sniper because of the pills throwing his aim off, having at least 2 engineers in this mission is a MUST! Scout can be good to slow him down and mark him for Engies because Engineers can't deal crit damage with sentries, only minicrit when they're marked or engies are buffed, Soldier can be very useful to stop Bannai from moving at all, this is easier said than done because you need to hit him directly, Demo is a no-go here unless you have good predictions, I'm not saying Demo is bad, but, he's not gonna be very useful here, Medic is a MUST! He has to revive and crit his team mates, or uber them, quick-fix is a big no-no here, Vaccinator is useless against melee, in Bannai's final stage, Explosive Resistance can be of use here.