So we haven't had any luck with downing new bosses recently... but we're getting there. We had some good success with Mimiron last night, despite some lack of strategy. I expect Mimiron will be bested in the next week or two.
We've earned ourselves a fair spate of a few of the more interesting achievements recently, at least, including our first "hard mode" kill of Flame Leviathan (only 1 tower up, but it's a start!).