Nothing you do is wrong. Dungeons are meant to be hard as it was the end game content years ago before the expansion and living world content. I recently started playing dungeons myself with max lege gear & infusions and got downed unexpectedly. The idea was that you learn how the mobs work and which to counter separately or avoid altogether if you want to save time. Your best tip is to go through dungeons slow the first few times to learn the basics and later you’ll breeze through as you know how to counter the creep and bosses. Like me, after 6 years even I had to relearn dungeon mechanics which I forgot after being too used to fractals, strikes and raid content.

Just keep at it untill you get the hang of the bosses. A group still stays more fun when done in cooperation. Now with even better elite classes you can have dedicated dmg dealers along with healers.