To target a specific area, you can do this:
<set xpath="//item[@name='meleeWpnKnucklesT3SteelKnuckles']/effect_group/triggered_effect[1]">
When you look at the Steel Knuckles in the items file, the triggered_effect[1] would change the code on the first one listed. [2] would change the code on the second one while [3] would change the 3rd one.