Clicker Heroes

Update:

As noted by Pawncrackers in the comments, this is NOT an experience level or upgrade level achievement. Grinding out gold to upgrade Cid does not unlock this specific achievement; it is based on the zone/area you have unlocked. Area #10, the second boss, to be exact.

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This is one of the very first achievements you should earn, unless you idle or grind out any of the other achievements first. Given the lower quantities of gold earned at low levels/areas, it does not make sense to stay at the start for very long.

This achievement unlocks as soon as the tenth area, Drylands Lvl 10, is unlocked. Areas are unlocked by defeating 10 regular enemies in a row during a normal area, or beating a boss (which appears every 5th area/level) within 30 seconds. If you leave an incomplete area (by accidentally pressing ), it will reset the counter and you will have to defeat 10 enemies from the start. Once 10 of 10 enemies are defeated, the next area is unlocked, and you can move back and forth among previous areas as you want. However, you can only continue further once you beaten 10 enemies in a row or defeated the boss in 30 seconds, depending on the area #.

Note that, once you beat the boss for Lvl 10, you will earn the in-game achievement "Zone Explorer". The Xbox achievement will come before this.