Might be a third time then. I've definitely knocked them down while they've been standing. You could always throw in a slow-mo missile somewhere in there. I just dodged a lot.
Having died a few times on the guy after them, I had a few tries at fighting them, and found that my best chance came from simply running around constantly firing off charged shots. I think they went down in three each, but with the diffusion effect it's ususlly a bit quicker.