Simple.
1. Create a trigger, make it the size of all the east units you want dead (If it's all on the map, just make it bigger than the map). Then, select 'East', and 'Not Present'. Make sure you're in Advanced mode, I don't think this works with Easy (To change, just click on where it says 'Easy' on the upper-right of the screen). Then, just put whatever you want to happen in the On Activation field. Or select End #1 to end the mission.
2. Name the two Shilkas as Shilka1 and Shilka2. Create a trigger, in the Condition field type:
! (Alive Shilka1) And ! (Alive Shilka2)
Again, put whatever you want to happen in the On Activation field.
This is not garunteed to work, you may have to delete the second !.
Beware the GrimMonkey