Method:
Most of the enemies killed will leave behind small gems. Blue ones are of the lowest value, then green, then red, and very occasionally you'll get even more coloured ones.
These gems are your experience - collect them to level up, as they'll allow you to collect new weapons during the run which will keep you alive for longer.
At the top of the screen is a timer - each level ends after 30 minutes. Survive in a single attempt for 20 minutes and you'll unlock the Pentagram.