Is WoL the only one able to master/handle all the classes at once?

[EdwinLi]

We see in the Shadowbringer Cinematic that the WoL can switch between his/her different jobs naturally through magical means so it is most likely a magical ability he/she has that allows for job switched.

Being a adventurer is a multi-skill job so it is common for adventurers to be skilled in multiple areas of talents. The creation of the Adventurer Guild was to not only open up a job opportunity for people with different types of talents but one of the major requirements for being a Adventurer is to be talented in multiple areas of crafts, gathering, and combat skills as mentioned by Louisoix when he was summarizing the events of 1.0 when he talked about the founding of the Adventurer Guild

While non-adventurers tend to focus on one job to specialize in for their daily life and earnings, Adventurers focus on having multiple skills for multiple "jobs" and "classes" for their different freelance jobs.