Auto-Cast Spell That Can Be Cast At Same Time By Multiple Units Onto Single Target?

I have several units that are generated and I need them to auto cast a spell on their way to a location. Like the way "slow" would react. I've tried using things like "cold arrows" or other similiar spells but the units have no base attack and i dont want them to attack. I just want them to cast and then continue with their order of attack-move to a location.

if i equip them with a spell like "slow" they do it almost like i would want but multiple units can't seem to cast on one target at the same time. It's like they each take turns to wait til the first one is done casting & and that duration has worn off.

I have the duration set to 0.01

i need the spell to spam from all of these units.. each having a cooldown of 1 sec on the spell.

but it's like the game says "hey, i see one unit is currently casting slow on a unit, lets hold off for the other units til he is finished casting"

That is exactly what the game says actually!
The game knows that with regular slow it would be a waste, so what you need is an autocasting no duration (or stacking) ability.

Try slow with 0 duration and no buff, and see how it works.

nope, it stays perm slow at 0 sec for some reason.

parasite seems to almost work but the computer wont make them use it.. it ignores using it without a seperate order.. which means they forget their original "attack-move" order. doh

You could always just re-issue the "attack-move" order via triggers after making them cast parasite. (I assume you already issue their attack-move anyways, right?)