how does the critical hit damagage multiplier work for camo cactus?

Hop into split screen along with a zombie and go to a safe place like the sewers to test it out.  You can do this with a friend in your yard instead of split screen, but I find it simplest to do this stuff on my own.  This is an easy way to determine these things.  In particular, this is a great way for testing out exactly what a plant character can do to a Z mech.  I guess in this case you'll need something with lots of hit points, like an all star.