I have two stories about the monsterous spiders of DAII.
I'm not an arachnophobe, but the first time I saw that thing in the Deep Roads I admit to yelling 'HOLY (STRING OF EXPLETIVES), WHAT IS THAT?" As it proceeded to chew through my warrior Hawke, Varric and Anders with ease, Bethany retreated up the stairs away from it's fangs ... and then realised that the monster spider couldn't climb after her! Not only was it too big to get up, it blocked the way of all its spider spawn. Bethany just cast Firestorm into the pit and stood there shooting at it with her staff until it died. I imagine that she never let her big sister forget that battle.
The other memorable time I had a run in with one of them, I was in a cave - don't remember which one, given their startling resemblance to one another. There was a whole mob of other spiders with it, and between keeping track of Hawke carving her way through them, Anders healing and Merrill raining lightning I lost track of the boss. Imagine my surprise when the last Poisonous Spider lay down and the battle ended. I turned around to find the corpse of the monster spider, with my dog standing next to it wagging his tail.
I think Varric probably helped him out, but I have never been so grateful for his tanking. Wished there was an ox bone to give him.