Ambush event is fairly simple. Keep your raid bunched up near the entrance gate of the event. Keep a constant flow of mobs coming to the camp but dont over pull yourself. You can simply let mobs just pile up down the tunnel there is no reason to agro every mob.
After 15.5 mins i believe it is, the named spawns. Kill the named and you win the event. Everything else will despawn once the named is dead thus again meaning dont over do yourself with mobs.
You will want to kill Murderers as soon as they spawn if they are in camp. They are pretty tough, yellow, and hit up to 3k max on average ac and shielding.