OP’s issue is not a Bot Improvements mod issue. It’s a vanilla game bot issue. See Allow bots to kill specials with ranged weapons
As per bots dying to hordes, I don’t think that’s a Bot Improvements mod issue too. It only does 3 things as per its description: better melee choices, stay closer, and ping elites that are attacking them. (All features can be disabled separately.) Improved horde fighting, dodging and blocking are not in the description. So if you are not helping them fight or give them poor weapons (e.g., low stamina, slow attack speed, low dodge distance and/or count, unoptimal melee combos), they will not fight as well.