Not sure how we can pull this off, but I've been thinking a bit about this game today, and it made me think of the simple game Fox and Geese. You probably know it, but as a reminder, I have provided a link to the rules:

http://www.mastersgames.com/rules/fox-geese-rules.htm

Though not a perfect analogy, the game we are playing is similar to Fox and Geese. We are facing a powerful fox (this time the fox has his own army of geese), while we outnumber him (and get two turns for his every one). As an uneven game, it is impossible to strike a perfect balance, and correct play should always allow the geese to win in Fox and Geese. One simply needs to cut off territory while protecting the geese from capture so that no new openings are made for the fox to escape to. In Kokusai Sannin shogi, we need to make our job easier by employing a similar strategy: we have to cut off his territory. Your dad will have to retreat towards his home base this next turn. What I suggest we perhaps start doing is playing a very safe, slow game, not really engaging in battle so much as reducing territory available to your dad. We should start at our common corner with defended pawn advances and proceed to push towards the Praetorian. We must be careful to guard our advancing pieces and prevent holes from being exploited.

Anyway, just a thought. Your dad is all over the place, and I don't know what else to do besides waiting for some horrendous blunder on his part.