Well actually, Sulla had already captured Athens and the Piraeus. Mithridates General, Archalus then retreated to Macedonia where he took command of an army numbering 100,000 infantry, 10,000 cavalry and 96 chariots. Sulla had only 15,000 infantry and 15,000 cavalry, thus he was badly outnumbered. But he divised a combination of both an ambush and a full frontal attack which created chaos for the Mithridatic army. But this was only a minor advantage as the enemies shear weight of numbers looked to topple the Romans. It was here that Sulla showed his brilliance as a commander as he went up and down the lines, diverting cohorts and commands to all areas of the action to constantly hold the line. Eventually he personally took control of the right wing and routed his adversaries while the left wing followed suit under Murena and Hortensius. What followed was a slaughter, only 10,000 of Archalus' men escaped alive. The Roman losses by comparison were light, in fact Sulla claimed he only lost 12 men, but this is hardly believable given the scope of the action.
Later that year, Sulla followed up his victory with another at Orchomenus. This time 60,000 of the enemy were killed and Mithridates was left without an army. The war from that point onwards was a simple affair.
As you can guess Chaeronea was my favorite of the above victories, Alesia been a close second. I would have added Ecnomus to that list also as it was Rome's greatest naval victory.