The Ghostflame Dragon (Moorth Highway) is an optional Field Boss introduced in Elden Ring: Shadow of the Erdtree. You don't need to defeat it to complete the game.
The Ancient Dragon-Man is the boss you can find at the end of the Dragon's Pit. The Dragon's Pit is accessible from the Pillar Path Waypoint Site of Grace by going south and taking the lower path ...
Much like Ancient Dragon-Man, Count Ymir ... losing without actively trying is difficult. Elden Ring is packed with tough boss fights, but beyond the main story ones, most are completely optional.
If you want to sign up to take part in the test, you can do so here via the Elden Ring Nightreign site. All Bandai Namco asks for is some contact info the confirmation of your preferred platform ...