This is missable. In order to get this quest, you MUST complete the main story quest "Blackpowder Courtesy", which can be picked up from the writing desk in your estate at the start of Act II. After competing this quest, go to Merrill's home and speak to her to begin her companion quest "Mirror Image".
NOTE: She must be in your active party in order for you to start this quest. Next, travel to Sundermount and talk to Keeper Marrethari to start "Honoring the Fallen". Take the path to the far left of the Keeper to enter the Varterral Hunting Grounds, where you will eventually encounter the Varterral.
Glitch warning: After doing the Mirror Image quest with Merril, avoid landing the final blow on the vartarral in the little cubby hole near the back of that room it's in. You won't be able to loot the beast for the Herbalist quest.
just wanted to add to #3 and #4. You can get this achievement early in act 2; you'll get a quest called "Blackpowder Courtesy". Complete this quest. In the process of doing so you'll get a bunch of companion quests. You'll get one for Merrill called "Mirror Image." This will take you back to Sundermount to see the Keeper. She will unlock the area where the 1st Varterral is. You can also complete that part of the Herbalist task quest at the same time.