This is a Jumping Puzzle
Try jumping on the platforms several times to cross the chasm and get the letter
You will most likely fail, as this puzzle is impossible
Tina: "Oh, what's that over there? It's a ruuune, at the end of a bunch of super-sick jumps and stuff."
Mordecai: "A jumping puzzle? Ugh."
After a few failures: Lilith: "I'm pretty sure this jump is impossible."
Tina: "Fiiiine, my bad. I'll make it doable.
After more failures:
Mordecai: "This sucks. I hate jumping puzzles."
Tina: "Okay. Whiny Wendy. A magical bridge extends, leading you directly to the rune. Are you happy?"
Mordecai: "Kinda, yeah."
Tina: "You found the dwarven letter 'A'."
Greedtooth: "Celebrate while you can, douche -- you still need two more letters to complete the passphrase, and I have the last one!"