One time I just casually strolled up to two poachers, and perfect protego'd the first ones attack. It killed them both instantly. Then I continued as if they never existed, and unlocked the door they were standing by. God I wish I'd recorded that!
It could be even better with the talent that shoot back 2 targets in case of perfect protego, and I'm not sure that you have activated the one which gives damage at stupefy. Usually everyone is dead before I can even start to really fight
I usually max out the core tree. If using dark magic it is ok to not use talents that boost ancient magic gain, because using avada kedavra asi much more satisfying.
I would definitely get the stupify talets. The perfect protego ones really depend on how often you can get off perfect protego. I would also get the talent which curses an enemy hit with stupify.
I don't have the one which gives more KO time after stupefy, because if you have the explosion of perfect protego the shoot back of 2 target at perfect protego and the stupefy that gives damage, the target is dead before anyway. Maybe it could be good on high level target.