For a long time, the seventeen Ranseian kingdoms were divided by two Warlords before you and Nobunaga. These Warriors were Shingen -- the modern master of Terrera -- and Kenshin -- the modern master of Illusio.
After some time, the two rivals decided that it was indeed too small a kingdom for two to hold power. Shingen names the challenge: the first to win five battles versus the other's kingdoms shall be named the winner.
Any walkthrough for this would be balancing on the head of a pin; if you lose one fight, everything will just go off-kilter and the guide will become useless from that point onwards. It cannot be expected for one to guide you perfectly through this; as there's no real sure-fire way to predict what opponents you'll be up against. As with all Pokémon games, learn weaknesses and examine your kingdom for good Pokémon to use in the next battle, be they recruits or otherwise.