Need help making monster spawn in 60min rotation

[behelit]

So, I have this simple script that spawns a mob... Then re spawns that mob after 1 hour has passed if you kill it:
 

-       script  sample_npc_mob  -1,{
OnTimer3600000: // 1hour
OnInit:
	monster "prontera",78,95,"--ja--",1153,1,strnpcinfo(3)+"::OnKill";
	end;
OnKill:
	initnpctimer;
	end;
}

How would I modify it so that instead of OnKill, it loops based on the following criteria: Start timer -> Spawn mob after 30min -> Kill mob after 60min ?

I only want the mob to be alive for 30min during the second half of the 1 hour loop.

Edited February 18, 2020 by behelit
Clarification

[Start_]

I think best way to keep it alive for 30min is don't spawn it.

But if you want to try, Look at this post: 

setunitdata UMOB_DMGIMMUNE.

Not sure skill can damaged mob or not but you can try.

 

[Litro Endemic]

why don't you make permanent spawn mob type? if you need a label, you can attach it on it too

 

[Emistry]

https://github.com/rathena/rathena/wiki/Permanent_Monster_Spawn