I find I have the same problem wioth Dragon's Age. I always do fine and upgrade my stats however the hell I want them, and then suddenly they toss a group of enemies at me that undermine all the stats I've put in. Said group, couldonyl be defeated if I were a tank, not a rogue.
Such is like the mage's tower, then you have to go into the faze. You have to kill these female faze things (Forgot the terms... Things seems appropriate). now when fighting one enemy, two, or three... I can usually hold my own. Here they asked me to hack on a woman wil qa significant amount of health while enemies regenerated all around her. With the number of times I was getting hit and the fact that I was on fire, I hit the heal button and he was too busy spazzing about before he could gulp down the potion.
Bombs, Dirty Fighting, and stealth help you hold your own as a rogue against tough fights in Dragon Age. The other skills and talents help to varying degrees because of the boss's creature class (darkspawn, dragon, etc.) will give them resistance or immunity to certain things. As for your example, well, have you considered using the other forms? I found that the golem and the arcane horror forms work pretty well.