That is defined in the simulation templates ( http://trac.wildfiregames.com/browser/ps/trunk/binaries/data/mods/public/simulation/templates/ )
F.e. the default attack of the archer is defined to be 6.5 pierce for the moment: http://trac.wildfiregames.com/browser/ps/trunk/binaries/data/mods/public/simulation/templates/template_unit_champion_infantry_archer.xml#L3
Note that templates inherit from each other. So you can define a default value for the generic archer, but define a special value for a special archer unit.
To redefine it in your mod, you must just copy the file to your mod, and change the values. Your file will override the version that's used in the game. F.e. in millenium A.D., they altered some resource templates to have other stats: https://github.com/0ADMods/millenniumad/tree/master/simulation/templates (in this case, they changed to colour of those elements on the minimap, which is also a stat).