Had to play against it a few times, and had to think how to counter it. 3k investment counters 2 ultimates at once, until seven buys Unstoppable, which is rare. Also useful against other channeled ultimates, when 7's is on CD, so it pays off.
Knockdown looks like a viable choice against 7 in most games (even without Ivy on their team) with a long cast range. It is something you consider to buy, once you see the draft.