After the third pair of wizards have come out (Happens around 70% transmatting), for the love of all that is holy, step out of the zone. Sincerely, Everyone who knows how to turn the public event heroic Edit: Killing the shriekers is what triggers the heroic event. If the transmat progresses enough, the shrieker basically opens and closes almost straightaway.
Honestly, activating the Heroic Event for the Warsat is one of the most brainless things in Warmind. First time I did it, I unintentionally activated it just by murdering everything.