Let me give you a scenario: Two Hydralisk, one supporting the other, are in a skirmish against a Wraith thats supported by a Science Vessel. Now lets say that the Hydralisk kill the wraith and the wraith kills one hydralisk, and no splash damage on combat cards. Is it impossible to kill the science vessel to claim the territory in this skirmish? Or does the science vessel have to retreat to another territory? Can somebody please explain to me what happens if you have an assist unit all by itsfelf at the end of a skirmish. Thanks!

If the Terran attacked, there are still Zerg forces in the area and the case is rather trivial (Terran attackers have to retreat) - so I guess you're asking about the case with the Zerg attacking. This is ruled on page 33 of the Starcraft rulebook:

"The only exception to this [meaning, attackers have to retreat if units from both sides survived the battle] is if all the remaining defending units are 'Assist' units [...], in which case the attacker is victorious and the defender must now retreat from the area."

So in your scenario, with the Zerg attacking, the Science Vessel is the sole remaining Terran unit, and since it's an assist unit (and thus all remaining defender units are assist units), it has to retreat.

Haslo has the right of it. Surviving assist units have to retreat. This could lead to the weird situation that you attack a sole assist unit with your own and win ^^

Thank you for clarifying! It makes much more sense to have the assist unit retreat. Thanks alot!