When you attack an assist unit alone with normal non-assist units, do you get to combat it and destroy it or will it just flee into oblivion untill you occupy each space making it unable to flee anymore and therefore get destroyed?

Both players must (if possible) resolve at least 1 skirmish in each battle. If you have no non-assist units, you have to assagn an Assist unit instead. It cannot destroy anything by combat values (no attack capabilities) but it migh be destroyed by the opponent.

It is even allowed to attack by sole Assist unit (e.g. to use Plague + Burrow or Spawn Broodling Technology cards).

BTW: Note that in this case your opponent can't use his Defensive module! This may be really usefull if you want to use "Mind Control" card effciently (although your Dark Archon will surely die, your can steal any enemy unit you wish - only EMP can protect the opponent).

Defender has to use any combatcard for his single Assist Unit. Probably a queen. He must use the default Combat valves.

If he play a Card from hand, he can use a spezial ability like broodling from queen.

If he play a default Card he can -not- use spezial abilitys.

U can read it in orginal Starcraft rulebook.