Darkheart Thicket is a pretty broken instance especially as Demon Hunter you can basicly skip directly to the endboss if you want to, you can also just decide which boss you want to do first doesn't matter in which order.
In this Video i basicly kill all the bosses in darkheart thicket in reverse order. You can see how it's done in the video it's to hard to explain good with my broken english
Darkheart Thicket Reverse Boss Order Speedrun - YouTube