An armor to edit could be: Scale armor, used for a set but the item alone sucks, sun cloak and light armor.
To apply the effect of reducing the enemy resistances, here's a template that you can use to insert into your items, just changing the % 4, 2 ,1 etc
Notice "belligerent=enemy" means the enemy gets the negative effect.
	Code:
	fight {
    {
      filter {
        belligerent=enemy
      }
      pbonus=
      dbonus=
      rbonus=physical,0,-4,0,-100,0,0
      attack_on=
      attack_off=
    }
    {
      filter {
        belligerent=enemy
      }
      pbonus=
      dbonus=
      rbonus=magic,0,-4,0,-100,0,0
      attack_on=
      attack_off=
    }
    {
      filter {
        belligerent=enemy
      }
      pbonus=
      dbonus=
      rbonus=fire,0,-4,0,-100,0,0
      attack_on=
      attack_off=
    }
    {
      filter {
        belligerent=enemy
      }
      pbonus=
      dbonus=
      rbonus=poison,0,-4,0,-100,0,0
      attack_on=
      attack_off=
    }
    {
      filter {
        belligerent=enemy
      }
      pbonus=
      dbonus=
      rbonus=astral,0,-4,0,-100,0,0
      attack_on=
      attack_off=
    }