Look at it this way. Doeg the Edomite didn't do it with his bare hands. He used weapons. Part of his weapons would include his son's and servants under his command. It's common to mention just the person in control in the Old Testament (Tanakh).
Note Doeg was "the chief of Saul’s herdsmen" (1 Sam. 21:7, ESV). Thus, he could have had help. However, within the context of 1 Sam. 22, King Saul and his whole army was around watching. So, with that protection it is feasible that Doeg could have killed 85 priests by himself. However, it would seem that he had some help under his command to kill the entire city of Nod. "he put to the sword" הִכָּ֣ה לְפִי־חֶ֔רֶב (1 Sam. 22:19) is an expression used for commanding to kill (Deut. 13:15-16).
Look at Abram and Lot. Their flocks with their servants where so large that they had to separate to different areas for the land to support their herds (Gen 13). Apparently Aram's servants made him equivalent to the city nations in that area.
Then one who had escaped came and told Abram the Hebrew, who was living by the oaks of Mamre the Amorite, brother of Eshcol and of Aner. These were allies of Abram. 14 When Abram heard that his kinsman had been taken captive, he led forth his trained men, born in his house, 318 of them, and went in pursuit as far as Dan. 15 And he divided his forces against them by night, he and his servants, and defeated them and pursued them to Hobah, north of Damascus. 16 Then he brought back all the possessions, and also brought back his kinsman Lot with his possessions, and the women and the people.
(Gen. 14:13–16, ESV)