The rule is:
<In order to clarify, the Sacrifice ability should read as follows:
A front-line unit with this ability (listed on its Faction Sheet)
is automatically destroyed during the Resolve Skirmishes
step of a battle regardless of whether or not the opponent
has sufficient strength to destroy it. This ability is ignored
if the unit with the sacrifice ability is unable to attack both
the opposing front line unit and supporting units (due to its
attack capability)>
But when an unit whit sacrifice is destroyed for his ability, destroy the opponent front-line unit ?
If no , whit an ONLY Sourge you can attack an enemy that have 1 fly-unit and a ground-unit... (then ground unit will is a supporting unit)
Then Sourge can't Sacrifice (can't attack the ground unit) but whit his combat card 9/6 can kill the opponent fly-unit...and if luckly can survive
If yes , whit a Sourge you can attack an opponet flying-unit and you can use a bad combat-card: the sacrifice ability destroy sourge and opponet flying unit...