Well quite a few people will come in here saying Focus attack it to break the shield or stop her from casting in the first place or any other aount of unhelpful advice. The only thing you can really do is:
A: hope you have a high enough DPS that you can focus HER and kill her shield and her. Trying to take on her allies with her shielding is a losing battle.
B: GTFO. scatter, make them split up. Go contest their flags in other places. Back up to your towers. Hide.
Basicaly, though it is sad, from what I have seen if you have 3 live players, one being a good QoT and the other 2 know how to play WITH a QoT, not much is going to stand in there way. Now an equal combo, such as sednas heal on your side will even things out, but short of that or something else equal to the bramble shield, your going to have an extremely tuff time.
I wish there was more mana burn items for this exact reason.