moon glaives
ok i managed to discover how to do this by myself, its actually quite easy if you follow exactly how the huntress works.
first, make a copy of the glaive attack ability and make it into a hero ability. you will notice that the hero ability does not have any fields that says number of targets. so, go back to the huntress and you realize that it has a maximim of 2 targets. it also uses an upgrade called moon glaives. so, make a copy of the upgrade and make sure your hero uses that upgrade.
now, you have to make the upgrade 3 tiers, you can alter the upgrade data to determine how many extra targets you get each time you upgrade.
after you done all these, you have to be a little more observant. you notice that there is no link between the upgrade and the hero ability. so, write a trigger, such that when the hero learns this specific ability, set the upgrade level of your copied moon glaive upgrade to 1 higher.
if you run your map now, you will realize that it may not work. this is probably because the upgrade is not available to your hero. go the scenario tab and click on upgrade properties. make sure your new upgrade is available to everyone.