The battle of Phaedriades (355 BC) was a Phocian victory early in the Third Sacred War, fought on the slopes of Mount Parnassus.
Diodorus appears to repeat himself in his description of the early part of the Third Sacred War. His account of the war starts in Book 16. In an account beginning at Diodorus 16.24.1 we learn that Philomelus was appointed commander of the Phocian army. He gained allies, seized Delphi and then defeated a Locrian invasion somewhere near Delhi. After this he hacked the judgement against the Phocians off the record slabs at Delhi, but promised not to plunder the treasures of the oracle. He then invaded Locris, where he camped near a river flowing past a stronghold. Assaults on the stronghold failed, and he then lost 20 men in a battle against the Locrians. The Locrians refused to return their bodies, claiming that they were temple robbers. This enraged the Phocians, who attacked and defeated the Locrians. They then raided Locris before returning to Delphi.
This was followed by a digression on the origins of the tripod of the oracle at Delphi.
Diodorus 16.28 then returns to the events of the war. Once again Philomelus gains allies, hires mercenaries, agrees not to loot the oracle, and leads his army to a victory over a Locrian army, this time near the cliffs of Phaedriades. He then leads his army into Locris and defeated the Locrians and Boeotians in a cavalry battle.
If this is indeed a duplicate account, then the fighting at Phaedriades was one of the earliest battles during the Third Sacred War. The war was triggered by a dispute within the Amphictyonic Council, which ended with the Phocians being condemned for cultivating land sacred to the Oracle at Delphi. The Phocians appointed Philomelus as their general. He visited Sparta to try and gain allies, and then took the drastic step of occupying the Oracle itself, which was located south of Mount Parnassus, and thus just to the south of Phocis. Although he didn't touch the sacred treasures at this point, Philomelus was still able to raise a powerful mercenary army.
The Phocians faced a powerful coalition supporting the League judgement, but their enemies failed to coordinate their actions. The first to move were the nearby Locrians, long term enemies of the Phocians. They got quite close to Delphi, as the resulting battle was fought near the cliffs of Phaedriades, on the southern slopes of Mount Parnassus. The battle ended as a Phocian victory. In the aftermath Philomelus forced some of the Locrians to throw themselves off the cliffs, perhaps mimicking a punished used against those who broke the rules of the oracle.
In the aftermath of this battle Diodorus repeats a section about the powers of Greece choosing sides in the dispute, before moving on to accounts of battles at Argolas and Neon. A possible timeline is thus that this battle at Phaedriades happened in 355 BC, before most of those powers had decided to commit themselves to the war. The Phocian victory would thus have drawn in the Thessalians, Boeotians and others on the League side, while the Athenians and Spartans joined the Phocians. The battles of 354 BC involved Thessalian, Boeotian and Spartan forces.