The game does not have an ending or kill screen, so getting a higher score depends mostly on surviving longer and clearing more screens. Riskier strategies for squeezing out extra points exist, but are less effective than safe conservative play.
Aside from bonus screens, which repeat after every third level, screen layouts only repeat after every 9th starting after the 12th. Each layout plays slightly differently, so you get a variety of different difficulties and strategies before any of them repeat.
The basics of the game is that you play on one screen, where enemies filter from the top to the bottom, to then come back from the top even angrier. The goal is to collect crates with the twist that a crate changes your weapon. Meaning you never know how each game is going to play out thanks to the