With which units can he be played?
Every barbarian unit can be played successfully with Arminius, but he is mainly designed for Cavalry. Infantry works pretty well with him too, although he is less sustainable in melee than Vercingetorix, for example.
What is his playstyle?/How to play with him?
Arminius relies a lot on Mobility to deal with his opponents. Frenzy allows you to deal a lot of damage in a short amount of time, so you can use Arminius for short engagements and flanking.
Infiltration makes your enemy see you as an ally (you become blue for him). You can use that to attack vulnerable skirmishers. It also gives you view range, which makes this ability good for scouting, but be aware that a single arrow can cancel it!
Momentum stops when you stop moving. So keep your unit moving to keep as much Momentum as possible.
Born in 18 BC into a Nobel family of the Cheruci tribe in Germania, Arminius was sent to Rome at a young age as a hostage to ensure peace.
While in Rome he learned military tactics and became commander of a cavalry unit. In the Pannonian wars in the Balkans, he commanded the auxiliary forces and lead with distinction. Arminius never questioned his loyalty to Rome but maybe he had no reason to before 7-8 AD when Rome began to increase their control in Germania.
While serving under general Quintilius Varius, in Germania, Arminius hatched his plan to betray Rome. In 9 AD Arminius got the Chauci tribe that lived some distance from the winter quarters, to revolt. Bringing the news to Varus himself, suggested sending three legions, with cavalry and auxiliaries, to put the Chauic back in their place. Familiar with the area, Arminius lied that he would guide the legions to Chauci territory.
To take a shortcut Arminius led Varius and the legions into the Teutoburg Forest where he sprung his trap. All three legions were destroyed, over 20.000 men with a few able to escape and all three legion standards were lost.
In 9-11 AD the general Tiberius led his army against the Germanic tribes but he failed to capture Arminius or recover the lost standards. Arminius organized the tribes into a single force under his control but they began to lose confidence in him.
In 14 AD the Roman commander Germanicus marched into the region, destroying tribe after tribe, burning their villages and fields, and showing no mercy. Germanicus was able to retrieve two of the three lost Eagle standards. When Arminius and Germanicus met in battle, Arminius was defeated and regrouped to hold Germanicus off a second time.
Tiberius who had become Emperor of Rome felt the war was over and recalled Germanicus in 17 AD.
Arminius who continued to rage war on Rome was murdered by his own people, who felt he had assumed too much power. Arminius however, is remembered as a hero and a champion for freedom.