Something which is absolutely necessary to making a shoot 'em up game like Resogun really enjoyable is enemy variance. This is what Resogun really nails with every one of the levels. Progressing through levels the enemies you blast through seem to go through an evolution of sorts, even just through the course of a single level. The mini-crabs which you tear through at the start of Decima gradually increase in size, begin hopping around higher, and are cleverly layered with other enemies making mere survival a tough task. Creating a variety of enemies, all with different speeds, attacking methods, and susceptibilities ensures that moment to moment action never gets boring.
The biggest contributing factor to keeping the game from losing its sense of direction is the continuous variance in objectives. Having to kill special types of enemies (called "keepers" - sometimes only on the screen for a short while, other times needing to be killed in a correct order) frees humans from glass boxes around the map which you need to pick up and take to safety for a small boost (just for a bit) to your weapon along with perhaps an extra life, bomb, points, or overdrive upgrade. Freeing all the humans on a level definitely isn't easy, especially because certain humans are only freed if you're keeping a high multiplier score. Resogun tries to make you play as efficiently as possible: always making sure you're targeting the right enemies, saving humans quickly, and maintaining your multiplier by continuously shooting at things while surviving.
As a result the gameplay feels frenetic but always about precision, keeping you active and hunting for enemies while making you wary not to waste table-turning resources like your boost, bombs, and overdrive. A lot very simple but well-crafted systems make the game fairly easy to understand and begin playing, but very difficult to master especially at the higher difficulties. Learning the smaller intricacies of the systems - like throwing humans while boosting to make them travel farther to their rescue point - makes for a cohesive and tight-playing experience which offers some spectacular and gratifying clutch moments of human-saving and enemy-blasting action.
The boss battles at the end of levels sadly lack a lot of these dynamic qualities which make the gameplay really shine. Often you'll be forced to peg away at most of them from a far distance while avoiding their waves of bullets. Rather than requiring quick decision making and the balancing of various abilities, boss battles are more of a rigorous exercise in focus and precise movement, changing the pace of the gameplay to border on tedious at lower difficulties, and frustrating at higher ones.
Despite boss battles which don't quite live up to the rest of the game, Housemarque's latest creation is still my most played PS4 title right now. For a game which cost me nothing (PS+ is a wonderful service), Resogun truly shines as one of the few launch titles which offers a deep and finely-tuned revision on the shoot 'em up style of game.