It wasn't immediately obvious what I was trying to do. At the start, left click did nothing (seems it was disabled before the first wave), but I got that soon enough. However, because the enemies didn't really recoil or otherwise clearly show that they were taking damage, I got sidetracked for a while thinking I had to destroy all the destructible environment bits.
I do like the wave characteristics being determined by the dice rolls; maybe having more than two would have been good, in order to create more potential combinations.