Help me with one Function

I try to make one function that "take unit U return unit"
The return unit:
is "random unit" within 800 of position of U and is enemy of U, not is U

I can't make it. Can someone help me !

BTW,
TriggerRegisterAnyUnitEventBJ(...)
=
action "Add Event to trigger()"

So number of Event in trigger that added will increase
Does this action make computer slow ?
And if i make com run slowly. Is there action can remove event from trigger to make computer run faster

Thanks

TriggerRegisterAnyUnitEventBJ(...) = Generic Unit Event