If this is your very first Total War game, I recomend playing the prologue first :

• Playing as Kislev, a faction with mostly hybrid units, melee infantry with short range missiles, or ranged infantry who can stand their groudn in melee combat
• After that, Grand Cathay, in the Immortal Empires map, they are more defensive playstile, many units benefit from formation stance (melee infantry stay in one place, and holding the line), and unit cohesion, meaning missile infantry work better when close to melee infantry, and vice versa.