"Cross treacherous chasm" is a mission objective in the quest Dwarven Allies in Tiny Tina's Assault on Dragon Keep DLC
Walkthrough
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 will use her Bunker Master to make the jump easier and easier until eventually she just creates a bridge which will allow you to reach the platform you needed.
Dialogue
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!"
