To get access to higher level units do this

Indian hunters 1: train 20 missile units and capture 5 regions
Indian hunters 2: train 50 missile units and capture 10 regions

Indian raiders 1: train 20 infantry units and win 5 battles
Indian raiders 2: train 50 infantry units and win 10 battles

Indian horses 1: 10 * defeat army with cavalry
Indian horses 2: 20 * defeat army with cavalry
Indian horses 3: 30 * defeat army with cavalry

Indian gunners 1: 10 * defeat army with gunpowder units
Indian gunners 2: 20 * defeat army with gunpowder units

Alphonse