Medieval synonym for outgun

Solution 1:

There are several words in Middle English that would generally convey what you like, but they aren't limited to equippage or armaments, like outpass and overweigh. Other Middle English words might be adaptable to the situation, like feten: one group may be feted fairer than the other, meaning that they have better equipment.

Since you aren't limited by that, I suggest outarmed. As defined by the Oxford English Dictionary:

transitive. To possess a more powerful weapon than; to exceed in possession or acquisition of weapons of war. Also reflexive: to provide oneself with more arms than a competitor.

1866 Spectator 14 July 773/1 At the Bistritz, as at Nachod and Trautenau, the Austrians were outnumbered as well as outarmed.

Though its first usage is in the 19th century, the cognate verb arm would have been associated with weapons from the 13th century onward:

a. transitive. To provide or supply (a person, army, ship, etc.) with weapons or military equipment in preparation for war or combat. Formerly also: †to provide (a knight, horse, etc.) with armour or mail (obsolete). Also reflexive.

c1300 Life & Martyrdom Thomas Becket (Harl. 2277) (1845) 2113 This lithere Kniȝtes armeden hem eftsone.

Since out- is a recognized prefix in Middle English (see also outpass), outarmed would be a plausible hypothetical coinage for a historical novel. And of course it makes sense to modern audiences.

Solution 2:

I think overmatched would be reasonable: 1 : to be more than a match for : defeat 2 : to match with a superior opponent