Hylek poison (https://wiki.guildwars2.com/wiki/Pot_Of_Hylek_Poison) has a large bug which allows players to remove invulnerability from enemies with it. So you can use all kind of abuse with NPCs that turn invulnerable such as Giganticus Lupicus and prevent him from going into the next phase.
One of my 30 accounts (Malediktus.9250).