Both enemies start with a small army, which can be crushed by the heavy infantry and elephant. Removing one of then from the game will make the fight much easier. Yellow is weaker, so he is more vulnerable. After removing one opponent from the game, build up a bit and tackle the other one.
Both the Red and Yellow government centers are within range of your navy - if you get close enough.
There are five Gaia Cavalry in the western corner. Unfortunately, they are stuck behind some trees, which will need to chopped down first.