I set a countdown Timer to 1800 seconds. Then i made some triggers for the timer -

Player "X"

Conditions: Countdown timer reaches exactly 1500 seconds

Create 25 SCVS at "SCvS" for Current player

============================

However, once the time comes, it never fires! Should i switch the trigger to Elapsed Time?

If you only want it to happen once, use "at most" instead of exactly. Or use elapsed time.

Or
Conditions:
Atleast 1495 Seconds
Atmost 1500 Seconds
---------
This gives the trigger a 5 second grace period to fire off
Downfall, it could be up to a 5 second delay which nobody wants to risk

pls delete / report this post.

BBcode again failed me, reposting helped. See below.

This isn't enough. If it should only run once you dont need the "Atleast 1495 Seconds", if it should run once each time the timer reaches 1500 it would run multiple times.
For 1 time execution do what LW said, if it may run more than once, add an action to Bol's trigger that sets a switch and a condition that checks for a cleared switch.
Then use an extra trigger that clears the switch for at most 1490 seconds.