Hi.
Just one suggestion I thought of.
If you had an event condition which was either one of the following:
If all of the X objects are destroyed...
or
If the number of X objects = 0...
Perhaps the second one would be more useful as you could utilise it in different ways.
Example: Say you have a wave based game and you destroy all the monsters, it would register as a satisfied condition and cause the next wave to spawn.