The game gives you two chances to kill a varterral.
Act 2:
You have to complete the mainquest "Blackpowder Courtesy" for Viscount Dumar in the Viscount's Keep. Shortly after you finished that one off Merrill's companion quest "Mirror Image" will become available. Talk to her and then take her to the Dalish Camp in Sundermount. Talk to Keeper Marethari and she will tell you about the varterral and give you a task. From there on you'll only have to follow the quest marker to find it.
The fight itself isn't really difficult. Watch out for the varterral's jump attack and the purple liquid he spits. Occasionally it will summon a few spiders.
Act 3:
In Act 3 there's a quest called "A murder of crows". It is only available if you have picked one of the game's pre built backgrounds when starting a new character or if you imported a save from Dragon Age: Origins in which Zevran is still alive.
If one of these criteria is met you have to talk to Nuncio in Hightown (Red Light district) to start the quest. It'll bring you to the exact same location as the one in Act 2.