As you advance through the game, you will be hiring heroes and purchasing levels for them in order to raise your DPS and defeat higher, better-paying monsters. As you buy levels, you will progress through the following in-game achievements, symbolized by a star:
* Guide (Level up heroes 100 times)
* Coach (Level up heroes 500 times)
* Teacher (Level up heroes 2,500 times)
* Mentor (Level up heroes 10,000 times)
* Levelupper (Level up heroes 25,000 times)
The achievement will unlock once Levelupper is unlocked.
Tracking of progress in-game can be done two ways:
1) Selecting Levelupper (it'll show progress as # of clicks and % to completion)
2) Going to the in-game stats page, which has a "Total Hero Levels " counter (tenth one down)
This tracking is done for your current run. Ascending will reset the counter back to zero, as you will start with no heroes and will have to re-hire them from the beginning. However, unlocking this without ascending will be close to impossible, aside from a heavy investment in either gem purchases or time. Ascensions provide Hero Souls, which act as a bonus multiplier to your DPS as well as acting as a currency to buy Ancients and level them up. This bonus and the Ancients will, over time, speed up the progress you make. Eventually, they'll allow you to make multiples of your highest-earned money in a fraction of the time.
As a side note, you will earn one Hero Soul per 2,000 hero levels upgraded when ascending. Killing a Centennial Boss (every 100 levels, likely once each ascension) will earn you additional hero souls to use, with more souls at higher levels. However, the most likely source of Souls are from Primal Bosses, encountered after Zone 100 and the first Centennial Boss. Every 5 zones, excluding every 100 / Centennial zones, has a chance of being flagged as a Primal Boss, identified by a note of how many souls they are worth and by a ring effect surrounding them. This flag is for the zone, and only affects the first time you kill the boss. Leaving and returning the zone, or failing to kill the boss will neither take away the primal flag nor assign it. If you kill the boss and claim the Hero Souls, then all future enemies at that zone during the current run will be regular bosses and not be Primal. Leaving and returning to a zone does not change a boss's primal state (neither on nor off). Basically, if you see a Primal boss, kill it for Hero souls, and if it's not Primal, try to get another 5 zones to try for another.
So to get to 25K, you will have to restart via ascension several times, getting farther and farther each attempt with the aid of new Hero Souls (+10% DPS each), Relics, Gilds, and Ancients. In general, I can get about 25K hero levels purchased around Zone 350. At this point, you can buy levels for the hero Dread Knight, and several heroes such as The Masked Samurai will be able to break the 1K Lvl mark.