HAWKED

To be clear, unlike the Stuntin' on 'Em achievement, this one does not limit the eligible knockdowns to one a match. It also counts knockdowns on people who have been revived.

What counts as environmental damage:

1. small red barrels (deal moderate damage - damage whoever you'll knock down to half their HP first)
2. spinning blades machine (can knockdown from full HP+shield - shoot at the crystal on the top of the machine)
3. big red barrels (deal small AoE explosion damage - whoever gets knocked down has to be low on HP and near the barrel)

What does NOT count as environmental damage:

1. floor spike plates (cannot be set off by a specific player)
2. pendulum blades (same as above)
3. floor fire plates (same as above)

You can generally find any of the eligible damage sources (most likely, at least two) at the main (named) parts of the map. I'd recommend cleaning out the AI monsters before, though. You don't want to have your kill taken away by a random AI, do you?