I'm getting the games set up for our Christmas party next weekend, and, as you will all know, Super Smash Bros Ultimate released today. Super Smash Bros is much more fun with all of the characters unlocked, so I would like to have them unlocked by then. I've heard you can unlock fighters by playing World of Light, but I've also heard that takes 20 hours to do. I don't have that kind of time. How can I quickly fill out my roster of 69 fighters and their echo fighters?
What I did was set up a 20 minute match and just play it and when its done you should get a new challenger. After you beat it, go to the home menu, close smash, then reopen it. This resets the 10 minute timer to a new challenger. Set up a match with one stock then immediately jump off the stage. You will then get a new challenger. After beating it, close the game, reopen and repeat.
TL;DR: If you're doing this with friends, the fastest way is to play short VS mode matches. If you're doing this by yourself, the fastest way is to beat Classic mode with every character.
IGN published a comprehensive unlock guide earlier this week, which details VS mode (and other playtime modes), Classic Mode, and World of Light as unlock methods.
According to the article, World of Light is the slowest way to unlock characters, and the fact that it unlocks characters at all is essentially a user-friendliness condolence. However, the unlock fights for the other two methods get progressively harder as you unlock more fighters, so WoL can be a good way to unlock the last few more easily, since not only do they keep their set difficulty level, you can also reattempt the fights there as many times as you like and can equip spirits to balance the fight in your favor.
The fastest way to unlock characters with multiple players is via playtime (VS is most common, but I've also gotten these fights on my way out of the Spirits modes). The game keeps track of how much players move their characters, which builds faster as you get more people. Once you get an unlock fight, however, you either have to wait 10 more minutes or close and reopen the game before you can unlock again. Characters unlocked this way follow a deterministic order, but not really any discernible pattern.
The fastest way to unlock characters by yourself is through Classic mode. Each Classic mode completion, regardless of the VS mode cool down, will present an unlock fight. This is also mostly deterministic - a few characters appear randomly, but most of the other characters are organized into "unlock paths" for each of the eight starting characters, in which the next character on the unlock path will present itself if you beat Classic Mode for the first time either with that starter character or anyone else on their path.
Note that with both of these latter two methods, the critical bit is being presented with the unlock fight, not actually winning it. If you fail an unlock fight, you reattempt it through the "Challenger Approach" mode, which appears intermittently on the Games and More screen as a set of double doors. In this mode, you can pick any character to do the fight with, so instead of being stuck with whoever you were playing as, you can approach the fight with your main or a hard counter to the character.