Well, Mithridates VI Eupator, or Mithridates the Great as he is also known, was a thorn in Rome's side for much of his life. He was king of Pontos, which was centered around the south coast of the Black Sea, mostly in modern-day Turkey. Under his rule, which lasted from 120 to 63 BCE, and under that of his father, Pontos grew to dominate the major part of Asia Minor. The Pontic kingdom at this time was a very interesting blend between the Greek and the Persian cultures (Mithridates was said by Pliny the Elder to have been able to speak and read the languages of all of his twenty-two nations).

Mithridates set off the first war with Rome when he ordered the massacre of eighty thousand Roman and Italian settlers in cities all over Anatolia. Here is one such scene described, taken from Adrienne Mayor's excellent biography of Mithridates:

When the violence began that day in Pergamon, thousands of terrified Roman families fled out of the city gates to the Temple of Asclepius. By ancient Greek custom, all temples were sacred, inviolable spaces, havens from war and violence, under the protection of the gods. Under the right of asylum (asylia), anyone—citizen, foreigner, slave, innocent or guilty—could find refuge inside a temple. Pursuers usually dared not commit the sacrilege of murder before the gods. But on this day, there was no mercy for the people crowding around the statues of the healing god. The Pergamenes burst into the sanctuary and shot down the trapped men, women, and children in cold blood, at close range with arrows.

This first war between Pontos and Rome was fought on Greek soil and ended with defeat for Mithridates, and with him being made to sign a rather unfavourable treaty. Two wars followed – collectively, these three wars are known as the Mithridatic Wars; they span the years 88 to 63 BCE. So all in all it took the Romans a period of twenty-five years, as well as the generalship of Pompey the Great during the Third War, to defeat Mithridates decisively and to allow Rome to assert its power over Asia Minor.

As I mentioned, Adrienne Mayor describes this well in her book, which book is named The Poison King after Mithridates' interest in poisons and antidotes (you may have heard of the terms mithridate or mithridatise). Here is another excerpt from it:

Mithradates became a legend in his own time. After the long Mithradatic Wars, even the Romans developed a grudging admiration for their most relentless enemy. Mithradates enjoyed a colorful afterlife in art, music, and literature. Medieval artists illustrated harrowing scenes from his reign, portraying him as a noble "dark Knight" battling cruel Roman tyrants. Machiavelli praised him as a valiant hero; his reign fascinated Louis XIV. Immortalized in a tragedy by the great French playwright Racine, Mithradates and his doomed harem also inspired the fourteen-year-old Mozart to write his first opera. Poets celebrated the King of Poison: "I tell the tale that I heard told. Mithridates, he died old." But even the details of Mithradates' last hours, death, and burial are shrouded in mystery.

If you are looking for a history of the actual battles fought in these wars, I suggest you pick up the book – Mayor goes into it in some detail.

Viriathus, the leader of the Lusithanians, was notorious for his guerilla war with the Romans. His resistance was so effective, Rome had to simply bribe two of his men to kill him in his sleep.

The best source for this confrontation is Appian, I think. http://www.livius.org/ap-ark/appian/appian_spain_13.html#%A761

Sertorius, though strictly speaking a Roman himself, held off Pompey with Spanish troops. Pompey never managed to get a victory over the guy, and only "won" when Sertorius' officers assassinated him. You can read about him in Plutarch's Life of Sertorius.

Sertorius used his Roman soldiers as officers for his Spanish infantry, and conducted his war in guerrilla style. Neither Metellus Pius nor Pompey was able to deal with this in any effective way. Sertorius was active for something around twenty years in Spain and Africa.

From my understanding of Plutarch's Life, what brought about his assassination was unrest among his officers who wanted to go home. Plutarch mentions two different conspiracies against the guy. They stabbed him to death at dinner.

Interestingly, Pompey burned all his papers after capturing his camp after his death. Makes you wonder what Pompey didn't want to be found.

He's one of my favorite characters in Roman history, and the one I will one day write a screen play or novel about.

Pyrrhus, the king of Epirus in around 300 BCE launched a unsuccessful invasion on Italy and Sicily. Though he won his battles against Rome, the Battle of Asculum heavily wounded Pyrrhus's army, giving rise to the term "Pyrrhic Victory". Rome quickly rebuilt its army and because of the Battle of Beneventum Pyrrhus decided to end his campaign in Italy. Hannibal Barca himself called Pyrrhus of Epirus the second best commander in history, Alexander the Great being of course being number one. One of Plutarch's Lives is about him. http://classics.mit.edu/Plutarch/pyrrhus.html