Personally I increase their Silhouette by 1, so the 5 Stormtroopers become a Silhouette 2 object. In the above example the FU could not have thrown that group without an extra Force Point or previously having purchased another Strength Upgrade. Had they been able to then the minion group would suffer 23-5=18 damage, taking out 3 of them and injuring a 4th, I would also allow that Triumph to remove 1 more Minion if the Player so wished.
Do you apply soak of minions only once and not once per new minion taking damage?