Forums for the Grievance Family

One method:
Play one of the base classes normally to the point where you start getting class specific sparks as drops from mobs.
Stop doing adventures and just quest and grind mobs in regional zones or in easy difficulty adventures (remembering to F11 out before completing it) to gain class specific sparks and use them to fill your classes atlas until you unlock the Ascension Atlas.
From that point on do missions/quests as normal and use the the colored sparks on the Ascension Atlas only, until you get to the class you want to unlock. Use only class specific sparks on the Atlas for that class.

Random Tips and Tricks:
Sparks earned from your capsule don't count against your spark cap. Every 3rd capsule will get you a bunch of sparks for free. So make sure you equip every class (even the ones you don't actively play) with gear as you get it to get some falsely inflated prestige numbers and quicker access to those good capsules.

Also, if you repeat Factory 501 until it's "Impossible" you can go in, kill the first boss, get your loot, and hit F11 to leave the mission. Takes maybe 5 minutes to do a run. The first boss is absolutely free. You can get a pretty good prestige buffer by farming him over and over. Useful if you're having trouble with more lucrative adventures or just want a little bit more prestige to unlock your next capsule.

If you're looking for a specific reward from adventures, check out the map, look at the rewards for a specific mission you have access to. If the reward is something you don't want, go to one of the regional zones (Isola Digs, Lanber Forest, etc.) and check the map there. The rewards will cycle every time you enter a different zone. That reward sucks too? Go to a different zone and check the map again. Repeat until you get exactly what you want. The 40 minute timer thing is completely irrelevant. Also works in a group. Once everyone cycles the reward to something they want, have one person enter the adventure and pull everyone else in and you'll all get exactly what you want. Unless you get bugged, but that's the price you pay for beta lol.

Just don't get too excited when you finally unlock the class you're working towards. They're pretty weak until you invest a bunch of sparks into them. You probably won't want to use them right away unless you did a lot of farming and all of your adventures are still easy.

Well it didn't take me long to get to 1k prestige and gain access to the training room to try any of the classes.