Extremely difficult....
Perhaps the following:
POD: Maodu of the Xiongnu is slain by his father Touaman of the Xiongnu when the former attempts to launch a coup upon his father after living in captivity with the Yuezhi-Gara.
Effect: The Yuezhi manage to keep the Xiongnu as a vassal state and conclude an existing alliance with the Han Dynasty in order to keep the Xiongnu, Wusun and Dong Hu in place. Yuezhi rule in Central Asia is assured for the short term and this does not permit the cascading invasions from the north.
Mithridates I continues his invasion of the Seleucid empire as otl and defeats Demetrius II. In Arachosia, the Eurcratides I is defeated in war by Menander I as otl, but his defeat is not followed by a fall of Bactria. Bactria remains a vassal of the Arsacid empire. Without the fall of the Arsacid vassal of Bactria, Mithridates I does not return east in order to see to the Saka or the Yuezhi. Instead, having captured Demetrius II and holding his life in his hand, Mithridates I in his last six years, launches an invasion of Syria with the intent of placing Demetrius II upon the throne of Syria and deposition of his brother Antiochus VII. This campaign is a major success as Mithridates I cuts a swath through Syria alongside an alliance with Egypt. Demetrius II is placed back on the throne in Antioch, however all lands from Duoro Europa until Damascus and Epiphaneia are occupied by the Arsacids and made into client states. The Seleucids themselves are made into vassals and Mithridates I marries his son Phraates to a Seleucid princess and perishes in 131 BCE. Phraates II succeeds him and with a Seleucid princes, attempts to maintain good relations with Demetrius II.
Phraates II perhaps puts down any rebellions in Bactria or keeps them as a good autonomous vassal. Meanwhile, House Suren, free from its disasters of otl, strike into the weakening Indian kingdom under Strato I and alongside the Bactrian kingdom, under a say, Diodotus III, are able to annex much land, with Bactria capturing Kashmir and the Surens capturing Arcahosia. For the next thirty years, the two chip away at India until they have formed a firm Indus border, concluding a friendly relation and division of India with themselves and the Shunga. To the north, the Arsacids already are the lords of the Kwarezm through their Saka allies, who will drift further into federation with the Arsacids in atl.
In the west, Phraates II focuses his efforts, attacking Armenia, Osroene and Adiabene. Allying with the Roman Republic against the Pontic kingdom and its ally, the Armenian kingdom comes with more benefits to the Arsacids, who are able to defeat both the Pontic state and the Armenians alongside the the Roman Republic, which gains the Bosporos, assured rule over the Aegean Sea and destroyed Pontus, a threat to their sovereignty. The Arsacids meanwhile move to make Armenia its vassal, which it completes by the end of the reign of Phraates II, who is succeeded by Mithridates II, who let us say crushes a rebellion in Syria under an Antiochus VIII and through his blood, claims the title of Seleucid emperor and settles his capitol at Seleucia.
This is the best way to restore the borders of the Persian empire without having to defeat the Roman Republic.
