I noticed you already saw my simple countdown timer example.
All you'd have to do is check if your objects are being destroyed.
for example:
Set TimeRemainingInSeconds to 15
Create a Global variable ObjectsDestroyed
on object destroyed add 1 to variable ObjectsDestroyed
system compare variable ObjectsDestroyed= 5
system trigger once
- system compare two values: TimeRemainingInSeconds > 0
-- system set text to : "Good"
- system compare two values: TimeRemainingInSeconds > 5
-- system set text to : "Great"
- system compare two values: TimeRemainingInSeconds > 10
-- system set text to : "Awesome"
- system compare two values: TimeRemainingInSeconds = 0
-- system set text to : "You failed!"