You heard quite wrong. Flourish and Snarble Barb were made for killing fiends (and beasts.) They will outperform any other sword in the game for that task.

The flourish series' first swipe is a wide arc and extremely fast. Good for making a hit then dodging or shielding immediately after. If you are up against many devilites, only use the first swipe not the full combo. Helps to lure one away so they are more manageable. There are two methods that work well for approaching them - first you can shield and walk slowly toward one until it attacks then kill, or you can run perpendicular to it until the devilite throws some junk which should safely go behind you then go in for the kill.

Against greavers (bats) you'll want to wait for them to show the attack warning, then turn around and swipe them to interrupt. This is particularly important in T2/3 where they leave a deadly, unshieldable status haze that can go through walls after they attack. Try to keep all the greavers on the same side so you can swipe at them all together with the wide arc. Again, only try to aggro one at a time for the best results (not always possible, especially if the team charges forward...)

As for the rest of the equipment... a shadow shield is really nice to have. Either Dread Skelly or Crest of Almire. And shadow defense equipment such as the Divine Veil works well, but isn't necessary once you learn their attack patterns. Divine Veil happens to also have a Damage Medium vs Fiends bonus, but the Divine Mantle does not.