Achie, welcome to 0 A.D.
That ticket sounds like something doable, but the description isn't completely correct either. Moving it to the Damage (or Armour) component won't help, as that component is part of the damage receiver. But the damage receiver isn't known until the arrow lands (the arrow has a pre-calculated path, but the unit can move out of that path). So you can't call the damage component of an unknown unit. Next to that, some projectiles (like ballistas) can deal damage to multiple targets.
The discussion on IRC is clearer in that regard: turn the damage helper into a system component, and make it keep track of the projectiles.
If you have questions, please join us on IRC (as bb said), then we can explain the structure of the project better, and perhaps show you some good JS resources.