If your not worried about being fair then once you begin on your army id recommend a split between empire and raven units.
Empire legionaires will hold a line against almost anything if you have them on a hill and close up ranks a couple of times aswell as packing 1 hell of a punch with there throwing spears but id suggest u only allow 1 volley at a time or they run out really fast.
Back that up with gladiators and you've got a very powerful shield wall backed up with one of the most dangerous melee infantry in the game and adding a few armored pikes can help especially when facing heavy mounted armies the hill slows the cavalry and a mixed wall of shields and pikes will stop a charge in its tracks and rip the cavalry apart.
Ravern soldiers its all about the archers the more you can get the better my army is currently some 300 men of which 180 are archers.
Then 60 legion 30 pike 20 gladiators who form my melee shield wall normally in 3 ranks with 3 ranks of archers behind on top of the hill plus my 10 companions all in heavy armor with whichever ranged weapon they have the best affinity for the finest chargers and a shield and axe.
It takes some time and id suggest getting in good with 1 nation first and getting business going couse when you drop from being a merc your income is gonna suffer if you havnt prepared for it in advance.
But with that army i just faced down and wiped out the main snake cult army of some 850 men.
I lost 200 of my own and not 1 melee soldier was on hes feet at the end but hey it was 3 waves before my first man went down and after that sheer weight of numbers overwhelmed my line as the damage built up.
But since im my main medic and have it maxed with a companion with it maxed of the 200 who went down almost 140 will live to fight another day and the empire can graciously offer me no reward at all for saving them from the invasion

being the hero is a thankless job but hey the rather large number of high teir prisoners the very nice gear and the 180 men i recruited all of which are high teir empire or ravern men im happy enough with the outcome and if the empire dont start treating me right ill have to take my own rewards beginning with locking the emperors ass in hes own dungeon and giving hes throne to the emperor in exile

.
Hope this helped.