Crash Landing Trophy in Baldur's Gate 3

1. Take the passage down into Wyrm's Rock Prison, near the Wyrm's Rock waypoint.
2. At the west end, turn both dragon-head torches blue with lightning damage (a spell, a scroll or an Arrow of Lightning). The wall between them opens.
3. Below, a statue sets you four trials: a fight, a chess puzzle, a riddle about who fails the city, and a judgement. Failing one only adds enemies.
4. Beyond them, touch Ansur's bones. The talk ends in the fight.

Kill the elementals first. When Ansur's health is low he takes to the air and gathers power for a big attack. Kill him in that phase. Globe of Invulnerability, from the spell or a scroll, protects the party while he charges.

Watch out

If he lands his attack before he dies, survive it and wait for him to gather power again.