Probably starting out with a Time - periodic event, every X seconds. then you create the amount of units you want at the region you want, give these units orders to attack move to the first region outwards and then, have a Unit enters region event trigger that check unit type of the units or player owner and then sends them to attack move to the next region in line and so on and so forth. So that you are kinda stringing them along, and units under attack move will fight any enemy or hostile in its path and if it survives the fight keep moving towards the original point. I hope this works for you.