If you are trying to count how many units are within the trigger, then you want the count command, not the forEach command. The syntax would be as follows:
"_x in (list trigger1)" count (units group1) == west countSide (units group1)
This will count how many units in group1 are in the list of units within trigger1's area. If that number equals the number of units alive in group1, then the trigger's condition is met. You can change the == to >, <, <=, or >= if you want. You can also change the west countSide (units group1) portion to some static number if you prefer.