It's been a while but if I remember correctly the Butcher has melee resistances so you'll need to fight him with ranged weapons in close proximity (like within 6 tiles) because he takes less damage from long range attacks. So masteries like Onslaught or Gunrunner are great for fighting him, but melee focused masteries and long range ones like Sniper not so much.

It is not Butcher that has too much armor, it is you who didn't noticed the "crippled" status you get after receiving a hit from him. Each stack of "crippled" makes you do 20% less damage, and you get full 5 stacks from single hit, fully neutering ALL your offense until it dissipates (provided that Butcher don't renews it with another hit). No surprise that your "6 ordinary hits" only did one damage each.

And, yes, you cannot attack him from edge of vision or beyond (nuking him with BFT out of sight is not an option), but starting from 5 or 4 tiles his "delirium" buff (damage and statuses resist) is no longer 100% (lowest is 20% at melee range).

Should have entered stealth, walked up to him and then gone nuts with assasinate or common melee attacks, Due to stealth, scouts have the easiest time against that boss.

Basically it is "quickly kill him from moderate to close distance without getting hit" or die, there is rarely a middle ground. But with you being allowed to bring a gun to this swordfight, he needs to have some sort of compensation.

Man, I admire people who understand so much the game, It was the same for me when I used to play DoomRL and in the Forum i dind´t get anything because all was too much for my poor brain, haha. Anyway, It´s great to know why this Butcher is so hard to beat in melee and long distance shots.

Every path in Dante except beeline to the exit and the Boss is a Hubris trap :P