Galaxian

Galaxian is a 1979 fixed arcade shooting gameby Namco and released by Midway in the US.

It presented a simple gameplay. Swarms of aliens attacked the player's ship. The ship could only have one shot on screen at a time and the player would defeat one swarm, only to have it replaced by another more aggressive and challenging in the next screen.

A version of the game can be unlocked in Mortal Kombat 3 on the Sega Genesis, that would be available by fighting in 2 player mode 100 times.