The shock trial (where you stun a bellowback and break off its underbelly canisters) can be done within a minute. It rewards 10,000 experience for completion and a box of blazing sun rewards, and can be done as many times as you like!
You need: A tripcaster with Shock Wire. A bow with tearblast arrows.
To do it: 1. Start the trial and jump down the rope. 2. You'll see a fire bellowback right in front of where the rope ends. 3. Set up the shock wire where the bellowback will walk through (or just right under his legs ) 4. Go behind the machine and aim the tearblast toward the underbelly. 5. One shot will open the canisters up 6. Second shot should be placed on the middle canister to have a higher chance to explode all three. 7. Run up and turn in the quest. 8. Profit.
Something to be weary about: watch the other machines around you. They'll sometimes make it difficult to shoot when they continuously ram you. If it ever passes the timer, or the bellowback gets up from the shock, just hit start and restart from last save. That'll put you back up top near the rope.
Ps. Not sure if anyone else has already put this on the Reddit, but I'm bored at work thinking about Horizon (and the Zelda that releases tonight!)