Infinite Sylvar Warrior Army, Gamebreaking (Patreon)

The bonus objective "The Fallen Tree" in the far east of Ardenweald, Tirna Noch, has an odd property. In this bonus objective there are many cages with sylvar warriors in. If you release them they will fight with you and help you complete the "fill up the bar" objective where you have to kill enthralled sylvar and Drust mobs. 

If you join any kind of group, including one with one of your own starter edition characters, and convert it to a raid, you can click on one of the cages and sylvar warriors will spawn infinitely, allowing you to command a large army which will take down any mobs regardless of their power in seconds. 

The sylvars are in assist mode-they will not attack anything unless you attack it. 

The sylvars will not follow you out of Tirna Noch, however the mobs in Tirna Noch are dense enough and re-spawn quickly enough that this isn't really a problem, and its a reasonably large area.

This is a very fun and fast way to level. It can also be used to skin mobs very quickly and make gold. 

The sylvars last only 5 minutes so you will need to periodically top them up at another cage. 

There is sometimes a world quest in the area which overwrites the bonus objective and removes access to the cages. Just complete the quest to get back to the right phase.

EDIT: now partially nerfed. You can no longer open cages in raid mode, though you can still build a small army in party mode. With good gear this is actually still pretty strong provided you have some self-healing. You have to switch between party and raid to acquire sylvars  and use them in combat.