<event>
    <action>
      <entry if_mob="zombie" action="a1 + a2">
      </entry>
    </action>
    <linked_action name="a1" if_world="world" action="set" sub="attack_power" value="yourdamage"/>
    <linked_action name="a2" if_world="world_nether" action="set" sub="attack_power" value="yourdamage"/>
</event>