Board Thread:Plants vs. Zombies 2: It's About Time/@comment-4775209-20130815185328/@comment-28857013-20131026084635

ProcastinatorMan wrote: There's no real strategy. Continuously replay levels, do the Star challenges and you'll get them eventually, or play Pyramid of Doom continuously.

Pyramid of Doom didn't work well for me, as I have done like 10 rounds and still no any single keys.