Unlock Talents quickly (Meta Progression)

Unlock all talents for your hero in ~ 20 minutes.

How to level quickly

• choose highest difficulty you have unlocked
  
• activate custom mode
  
• set negative modifiers: Berserker Foes, Angry Birds, Blood Lust, Disease and Inflation/Dried-up Wells (do not pick Oppressive Nightmare or Shadowy Fog)
  
• start the run
  
• immediately head towards the boss
  
• defeat some minor enemies on the way to gain xp
  
• start the boss fight by standing inside the boss area once you're there
  
• once the boss fight started, abandon the run using the menu

(Note that the time in this screenshot is 55 seconds, not minutes.)

How long does it take to reach lvl 9?

Reaching lvl 9 takes 3,600 xp. One run gives a bit more over 300 xp.
Reaching lvl 9 takes about 3,600/300 = 12 runs.

Each run takes roughly roughly 1 - 1.5 minutes. With about 20 seconds in between runs for loading times etc. each run effectively takes about 80 to 110 seconds.
12 * 80 / 60 = 16 minutes as lower bound
12 * 110 / 60 = 22 minutes as upper bound

=> Reaching lvl 9 takes about 16 to 22 minutes.

Coop

If you're playing with a friend, you can get even more experience.
1) Simply by playing multiplayer a small xp multiplier is added.
2) One player can run towards the boss while the other fights, so the run is quicker, maximizing the bonus from "Early Awakening of the Boss" while still getting some xp.

Details

This strategy mostly works because of the "Early awakening of the boss" xp multiplier. Getting to the boss early is key. The other big factor is xp. Try to get xp while on the way to the boss as possible. If you have stealth and can sneak a watch tower, go for it.

Negative modifiers increase the xp gained. Here's why I choose these specific modifiers:
Modifers give +10 or 15% xp, obvisouly you want to pick as many +15% as possible.
Berserker Foes, Angry Birds, Blood Lust and Disease give +15%xp
Oppressive Nightmare and Shadowy Fog also give +15xp but they do more harm than good. Oppressive nightmare reduces the "Early awakening of the boss" bonus, shadowy fog hides the map so you can't find the boss quickly.

Having chosen 4 out of the 5 modifers, only +10% modifiers are left.
Invigorating Death makes it harder to farm xp. Lack of Inspiration removes 1 starter talent from your options, but most heroes want to use a specific talent when doing these xp farm runs. Corruption makes all enemies corrupted, meaning they are way more tankier and dangerous but don't give more xp.
This leaves only inflation and dried-up fountains as valid options.