Two of the three hidden characters in Dragon Ball FighterZ are hidden behind the arcade mode. You can unlock them by earning zenny in the main story mode, character trials, and playing some matches in versus mode or online, but the fastest way to do unlock SSGSS Goku and Vegeta is to beat the arcade mode's two toughest challenges. To that end, we've put together a short guide to make sure you're busting out times-10 Kaioken Kamehamehas as quickly as possible.
First, unlock the hard versions of the Extreme Gravity Spaceship (for SSGSS Vegeta) and Hyperbolic Time Chamber (for SSGSS Goku) courses by beating them on normal difficulty. This shouldn't be too tough, as the A.I. won't bother you too much until some of the later fights (and if you're stuck on those, the other tips in this guide should also help).
Once you've done that, you'll have to beat the hard versions with at least an A-Rank. You're ranked after match (From D to S), and, depending on your rank, you'll move up, down, or forward through the arcade-mode tree to face a different opponent. The faster you finish a match and more health you have at the end, the better you'll do. The opponent you face at the end is roughly indicative of your what rank you're likely to get; if your final opponent is at the bottom of the tree, you probably won't get an A rank, and if they're at the top, you'll probably get an A or S rank. So if you're dragging across the bottom of the tree for most of your run, you're better off restarting.
Because your opponents in Arcade mode deal much higher damage than you (up to 80 percent of your health bar for a single level-three super toward the end), you need to minimize the amount of openings they get on you. The best way do this is stay in the air as much as possible, as I found dealing with grounded mixups from A.I. opponents tough to deal with at times.
It's much easier jump backward and spam Super Dashes from afar since, for whatever reason, the A.I. seems extremely susceptible to them. Very rarely would they block a super dash from afar, though close-range super dashes were less successful. To maximize the damage you get off a successful super dash, for a quick magic series (light > medium > heavy) except you'll want to finish with a down+heavy attack, which will launch the opponent higher into the air.
If you press forward and the assist at the same time, one of your teammates will come in to do their own super dash, which will let you continue the combo with another magic series. Finally, if your opponent has just a sliver of health left, feel free to fire off an airborne super.
This is the main strategy: Super dash as much as possible, then use upward launchers and swaps to extend these openings and avoid getting hit. Keep in mind, however, that your ability to swap characters mid-combo is on a timer, which means you'll have to alternate who you swap in and out (you can keep track of these timers by looking at your on-screen character icons) and occasionally wait a bit to do your next combo.
Team composition is also important. The most useful characters for this strategy are those who have level one and three supers they can fire off while airborne, which is why I chose Super Saiyan Vegeta, Android 18, and Kid Buu, all of who fill these roles nicely. Not every character's down+heavy attack launches opponents upward, either, so keep that in mind.
Finally, if your opponent gets low without you finishing them off, you'll want to swap them back in, since that will remove their stock of blue health (health they recover while not in play). You can do this by going for a throw, then holding down the corresponding assist button (L1 for the second slot, L2 for the third slot by default) as you're throwing them. Conversely, you'll want to swap out characters with lots of blue health; although arcade mode opponents deal more damage, they also leave characters with more blue wealth, which makes swapping characters out of combat to heal them crucial to getting a higher rank. Don't focus on this too much, however, as throws and swaps can be risky when thrown out at random and can get your characters killed in an instant during later matches.
These tips won't make you a better player, since they're mostly about exploiting A.I. (anyone with common sense would start looking out for super dashes), but they should help you unlock SSGSS Goku and Vegeta. These modes can still be tough and this may take a few tries (and some luck) depending on your expertise, but hopefully, these tips will make the whole process a bit less frustrating. You can watch a video of an S-rank clear of the Hyperbolic Time Chamber course above to see these tips in action.