Hi Amonicane,
in the same way you change the attack value of your new archer, you can change the attack and bonus values of your secondary projectile, simply by selecting the projectile unit ID instead of the archer ID, and adding attack values in the projectiles unit ID attack values tab.
Hope it is useful, i'm glad there are still new peoples approaching the modding!
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MY GAME VERSION:
Age of Empires II The Conquerors 1.0 without any patches, just the ES release in August 24th 2000, proud to playing this game version!