Massilian movement in Solo - offiicially does not exist. Massilians in Solo do not follow the regular sequence of play - they only get x attacks/fights depending on Caesarians' activity. I have not tested it extensively - in my games I was not able to eliminate a single die (because of the modifiers). Slow trickle of losses, sure... so I have never ended up with a situation where Massilian move was crucial. Theoretically, advancing to the weakest target may end up in a move (into adjacent space to the target)... but this would need to be stated in the postcard more clearly.
All of this may be problematic in solo mode - but at least for now, I do not have anything better as an answer.