mkay. what you need is a trigger covering the whole map, activated by whichever side you want to do the chasing. in the 'on activation' box of this trigger you should put
all_enemies = thislist
now, when you want them all to start attacking the unfortunate unit, you need to run a script to get them all moving. as thobson said, repeating the script every 30 seconds or so is vital. if you run it only once, they will move to where the unit first was. the unit could have moved on by then, so you need an update of his position.
the script would look something like this
;all enemies chase unit
#loop
? not (alive unit_name):exit
{_x domove getpos unit_name} foreach all_enemies
~30
?alive unit_name:goto "loop"
and just for good measure, i've put all this into a demomission so you can see it in action. good luckÂ