The Wiki on Levels (including rewards): https://hearthstone.gamepedia.com/Level
Each hero class has a ‘Level’ as seen here in the bottom left of the hero portrait along with the XP bar. (Note that with Nov. 12th, you’ll have two forms of XP – one for class levels which are just cosmetic, and one is a progression that is taking place of Daily Quests and Weekly Quests and the Reward Track)
Why Power Level?
1) There are probably a few achievements tied to this coming in the Nov. 12th update.
2) You get golden Basic cards. Some can be pretty good and do see play and nice to have in your collection.
3) Potential free dust with the Hall of Fame rotation. The Hall of Fame this time around is looking to shuffle a few Demon Hunter cards as well as make a few changes to other classes. This can involve sending some Basic cards into the Hall of Fame to replace with something else. Getting Demon Hunter to 60 seems very fruitful because your golden Basic cards will now become Golden Commons and you can disenchant them for free dust.
How Gaining XP works
Play a card (3 xp)
Kill an enemy minion (3 xp)
Kill your opponent (30 xp)
Play a game for 3 complete turns (3xp) (note caveat mentioned later – you have to play something)
And a bit more
TLDR; play lots of cards, kill lots of minions, kill the opponent.
2) Experience is gained everywhere even if you lose
Number of turns do not give XP
Amount of time (minutes, hours etc.) do not give XP
If one player plays normally and gets normal XP, and the other player does absolutely nothing and will get nothing.
You can’t get more than one Level up per game, excessive XP is saved.
5) Other XP Notes
XP from Level 1-5 works differently, as opposed to Level 6-60.
Demon Hunter starts at Level 20
We don’t necessarily have to win every game, but being able to play a lot of cards, and destroy a lot of minions is still enough. Ideally we assemble some cards like Echo or Repeatables and minions that spawn more minions, destroy them, and repeat until we hit a threshold and get enough of a level, to then concede.
The same strategy is probably not going to work for every class, but we can assemble them into two types:
We can use solo adventures to our advantage and identify some adventure missions which can be completed quickly and give lots of XP (spawns many minions, play many cards, destroy many cards etc.)
The decks are probably going change for each class depending on the encounter.
If you don’t have a large enough collection, some adventures like Dungeon Runs can work. Though notably some of the class XP gets weird in Tombs of Terror (e.g. I play Finley, the Shaman and Paladin, and neither seem to be getting XP). The original Kobolds and Catacombs still work.
I can also identify that in Blackrock Mountain, the Mage class challenge is quite decent. You are playing Unstable Portal multiple times, which counts as playing two cards.
2) With a buddy
If a willing buddy (or get a second account and play simultaneously) can let you, you can setup a few nice turns to grab lots of XP.
Perhaps a Defile Patron setup for Warlocks? The turn will eventually end after some amount of actions, but not before a lot of minions die, and you can setup the Patrons by your opponent to clear many minions.
3) Get XP as you go along
The only real ‘deadline’ is for Hall of Fame 2021, which arrives sometime in early Spring 2021 (not confirmed but follows a trend as seen here). That’s approximately ~5 months so you can take it at your leisure and double count and progress other goals while doing this one.
I imagine there are still people who have some empty XP slots for their classes interested in speeding it up. I have most of my classes (except Mage) at 50, and my DH is at 30.
Some Tavern Brawls can be pretty good if they have you play lots of cards and then you can both earn XP and perhaps grab wins and the 10 gold per 3 wins.
I imagine trying to boost XP would be much harder to pull off in rated matchmaking since the competition is much higher. It would probably involve having cards that generate more cards and clearing lots of minions.
Why Work in Progress
Much of this research by the community is pretty old, with some dating back to 7 years back in 2013 when the game was in Alpha. I couldn’t find more updated information so I wanted to find:
Whether there is an updated guide.
Whether someone took a look at the current collection, both Standard and Wild and saw which ones could be used to boost XP.
Double checking info in case of hotfixes or changes to experience since 2013.
Get some useful info for myself.