No sweat:
1. Use spies from your foothold Mongol city to acquire the techs you lack.
2. Go back to Demo and build up your trade routes to maximize money and science production.
3. Boost your cities to max size with WLTP days.
4. Keep your tech and income rolling in until you get to Robotics. THEN you switch to Fundy and produce lots of howitzers, mech inf, and armor.
5. Begin the invasion.
6. When the Mongols have been sufficiently weakened, end the war and return to Democracy.
7. Gloat.
Have fun and be careful with that a*s!