These include things like beating the level in a certain time, having a certain number of head shots, keeping your hero's health above a certain threshold among others. (brokenjoysticks.net)
A formula has endured for decades that players inherently understand and accept in principle; after beating a level you face off with some of the hardest video game bosses. (igcritic.com)
It's hard to beat that level of convenience. (geekwrapped.com)