Use this in a trigger
("Alive _x" count units bravo) <= x
PUt instead of x the number of man that will be alive when the 80% is dead. I mean, if there are 10 men, put there 2, so 8 or more will be dead when the trigger gets activated.
Now in its On Activation field put final=true.
Now create another trigger, Condition final, with the end you want