im sorry, i feel very, very noob saying this but i need a little help on how to make a periodic spawn trigger. Well, first of all i said

Cond.: Switch 1 is set
Act.: Create 15 uruk hai at spawn 1 for player 5
clear Switch 1
Preserve trigger

TRIGGER 2:
Always
Act.: Wait 30000 miliseconds (30 sec)
Set switch 1
preserve trigger

I dont want to write it in the form "elapsed time is " 30 seconds...and shit like that...itll take too long....

Please welp...

that should work, but you cannot have any other wait actions running at the same time for the players owning these spawn triggers, or you will get wait blocks.

if these players need other waits, then use death counters instead (these death amounts assume you have hyper triggers)

example:

C:
p5 has suffered at most 359 deaths of goliath turret
A:
preserve trigger
add one death of goliath turret for p5

C:
p5 has suffered exactly 360 deaths of goliath turret
A:
preserve trigger
set deaths of goliath turret to 0 for p5
create 15 uruk hai at spawn 1 for player 5

with hyper triggers, triggers will execute 12 times every second (realtime), so 360 deaths ends up as 30 seconds, which is the equivalent of your wait(30000).
if you do not have hyper triggers, you will have much less deaths, but i'm not sure how fast they run exactly.

It's closer to being exactly 30 seconds with 357 deaths, if you don't have hyper triggers it will be exactly 15 deaths.