In my opinion, fire is a counter for both MK3 Hammers and bad aim, since it raises the PTK of weapons by removing extra voxels, which may make that arrow that struck the robot in the middle of its chest chop the same robot in half, and it's also a counter for (sigh) Combine Archers, since they are so... I don't know, "blocky", or "defensive" :P, also, are they supposed to block arrows with a bow? I saw a MK3 Combine do that.
Disclaimer: the next few pieces of reply are only MY opinion, if you don't agree with them, don't burn me at the stake, viewer discretion advised. Why did I say that?
I don't really use jetpack for combat, ever since the hammer was nerfed, I use it more for transportation and dodging, and also to have a speck of mobility when a leg is chopped off.
The kick is not only a counter for MK2 Jetpacks, but also the holy grail of bow only challenge and also an ability to get you some thinking time and possibly an execution (Which is what I call kicking a robot to the ground and killing it while grounded) When I play endless I even neglect getting a clone as first upgrade and instead settle for the almighty kick.