u/ButerU_KR

1. Place any companion character near Varnia.

2. Switch to another character and teleport to Sunset Valley.

3. Summon your horse and ride it for a bit to trigger the challenge countdown.

4. Once the countdown starts, switch back to the character you left near Varnia.

5. Immediately mount your horse and enter Varnia to finish the challenge.

P.S. You can use this exact same method to clear challenges #6 and #9 as well

Shoutout to Pearl Abyss for putting "Summon," "Dismiss," and "Character Swap" all on the exact same UI. This tip was born from my absolute misery, so enjoy.