Arminius was smart, Boudicca was stupid

Arminius was intelligent when it came to fighting Rome. His years as a hostage and serving in the Roman Army meant he knew how the Romans fought. He therefore forced the Romans into battle on ground of his choice, in the forest where the legions could form up into formation. He also wore the Roman down with two days of hit and run attacks and fought from behind a wall, so the Roman legions could not overrun his army. As a result he annihilated three Roman legions and Rome never attempted to occupy Germany again.
Boudicca, on the other hand attack the Roman forces on ground chosen by the Roman commander. The Roman force was deployed between two wooded areas so they could not be outflanked and woods behind so he could not be attacked in the rear. This deployment also took away the numerical advantage Boudicca's army had. The British attacks were repulsed by volleys of javelins and the legions shields and swords, and the Romans then advanced and killed 80,000 Briton, crushing the rebellion. Britain remained part of the Roman Empire for another 350 years.