Sometimes you may need to hunt down specific types of Destiny 2 enemies. This may be for a quest, bounties, or limited time events that occur in the game. Season of the Worthy introduced Rasputin bounties, one of which is to find powerful fallen enemies. Strong enemies are above a red health bar, and finding them is actually pretty straightforward.
Public events
Fallen Public Events, which take place on Earth and in Nessus, are a great place to find powerful Fallen enemies. You have to kill ten in total for the powerful Fallen Rage Bounty, and a public event like Glimmer Harvest will allow you to do that easily. The first two waves of enemies will have strong lees and lesser enemies, very easy to kill. After their death, a powerful captain will arrive, adding another notch to your belt. You can get the whole thing wrapped up in two swift waves during this event.
If you want to use your time efficiently, you can try the new Seraph Tower event near the Winding Cove in the EDZ. It will throw waves of Fallen at you, contain many powerful enemies for you to take down.
Gambit
If you play Gambit regularly, you can find Powerful Fallen whenever they become the enemy horde in this mode. It's not ideal, as you never know what kind of enemy you're going to fight, but if you're not in a rush and play Gambit a lot anyway, this is a solid way to find them.
Lost Sectors in Trostland
Trostland in the EDZ has three Lost Sectors near the Church, all of which contain powerful fallen enemies. This is probably the slowest way to grow them, but it's an option for those who need it.