DR, charm effects, make the gun an unsafe choice due to the setting, give the other players a situational advantage like a fixed or crew-serviced weapon, make the fight more of a puzzle than pure action, give the fight stages where different types of damage are necessary, let the fight itself be avoided or changed with diplomacy.

Hell, don't worry about it. Exploration, interaction, and combat are all equally important in a successful tabletop game; and if they aren’t equal, there is a reason combat comes third in the list.