[Johnny1A.2]

This isn't a universal solution, but many of the major robot models in canon RoS will have problems with certain sorts of terrain and environment. For ex, a Centurion is ill-adapted to jungle or swampland. So as a tactical matter, it would be worthwhile for the humans to try and force the robots to confront them in such conditions, where they have a relative advantage. (Humans don't do well in swamp either, but we're better suited for it than Juggernauts.)

As an addendum, by the RAW of canon RoS, the bigger robots tend to be smarter. The human-level (intelligence wise) robots are the Juggernauts and Centurions and the like, most of the human-shaped or human-sized robots average around IQ 9. That means if you can mess up robot communications somehow, in environments where the big machines can't go, the human-sized ones can be outsmarted by human forces fairly plausibly, the moreso because the nature of the setting means that low-intelligence, low ability guerillas tend to be culled quickly.

The upshot in that a unit of 20 humans may well be more intelligent on average than a unit of 20 human-sized combat robots.