By what i read, this line already will work as a waiting time before your event will start:
// Announce event will start soon
announce "[World Boss] A rift on time and space is about to be opened! A World Boss will appear in " + (.event_sleep / 60000) + " minute! Prepare your Team!", bc_all, 0xFF0000;
// Sleep until X time have passed and then continue script
sleep .event_sleep;
So if you want it to count from like 5 to 0 seconds before it starts, try adding this after the "sleep .event_sleep;" line:
for(.i==5;.i>=1;.i--){
announce "[World Boss] A World Boss will appear in " +.@i+ " seconds! ", bc_all, 0xFF0000;
sleep 1000;
}
Hope it works ?