Count the number of east units at the beginning of a mission, save into a variable.
Then have a trigger check for:
?(enemyUnits < 95)
then use that to end the mission.
(95 was assuming you had 100 enemy soldiers, change as appropriate)
I think this is a little ambiguous. To be more clear, in the trigger's Condition field, put the following code:
count (list this) <= x
Where
x is the number of remaining East soldiers that would be only 5% remaining (i.e., 95% are dead).
Thus, if you had 20 east soldiers,
x would be 1 in order to check if 95% are dead. Your code would thus be as follows:
count (list this) <= 1
The list command provides an array of every unit in the trigger's area that would activate the trigger. Thus, this code counts how many units would activate the trigger, and if there is less than or equal to 1 (95% dead), the trigger will be activated.
Good luck!