ok, you set it up in triggers for the game to create x ammount of units in a region, regions can be made off of the pallete you have on the right hand of your screen in the world editor. Just use the pull down and switch to region editor. Make a region where you want the units to spawn first. then, make another region at the place where you want the units to go to make the players lose life.
In trigger editor, you want to put a new trigger in, the cause trigger(cant remember its name! its the flag one i think) needs to be a unit enters a region and the region needs to be the one your units spawn in.
Then, in conditions, make a player comparison where the units are equal to the player that controls the units(so you would need to make an extra player to be that character OR use hostile creatures but they tend to fall asleep when night falls).
then make an effect(i think thats its name) that is issue unit order to target a point and you want it to target the point where the player loses life.
now, if your map has curves and turns in it, this will be a bit more complicated. You will want to have the same start up as where the units spawn in the first region, but instead of telling them to walk/hover/run/fly strait to the lose life region, you will need to make regions at the curves or turns. Otherwise your units will get stuck and it wont be a fun map
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ok, lemme break it down in trigger menu.
Trigger->Create new->Cause=time
eriodic(if you want units to spawn periodicly)->create units x ammount of x type for x player
then
Trigger->Create new->Cause=Unit enters x region->condition=player comparison unit type is of neutral hostile or another player->Issue triggering unit to move to x region,
did that help? if you need anything else please dont hesitate to ask.