Thieves used to be able to backstab with the fists before ee but as has been stated already, it’s kind of a joke and have not used it in a looooong time. Assassin dualed to cleric backstabbing with BMU damage is the highest damage available with one hit in the game I think.
The BMU's ammunition is actually phenomenal when paired with the Firetooth Crossbow and an archer- it strikes twice, so you get your Called Shot benefits twice. Note that it strikes as a low level magic item, but the Scorcher Ammunition, at least, hits anyone in its path without checking. Stick a Skeleton Warrior on the far of an enemy you want dead and attack it with the Scorcher Ammunition/Called Shot.
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