This is a twin-stick shooter. Left stick moves, right stick aims, holding L2 toggles a laser sight that reduces bullet spread at the cost of slower movement, R2 fires, L1 uses your special item (different for each character), the X button reloads your gun (which takes a solid 1-2 seconds, double if you’re using a mid or heavy gun), Y swaps between the two guns you have equipped, A dashes, and B does a melee attack (which you’ll need if you run out of ammo). You have to kill all the enemies in a level to progress to the next one (except the final level, where you just have to kill the final boss). Also, there’s a small chance that a random relic will be detected once you beat a level; collect three in the same set and you can activate their effect on your next run.
As is typical for shooters, enemy projectiles spawn with no warning, so you need to react in the time it takes for them to reach you. Thing is, some enemy weapons (like the spread gun) fire so quickly that you won’t have time to move out of the way unless you dash, but since dashing is mapped to a face button in this twin-stick shooter, you won’t be able to push it in time, effectively making it useless. You could theoretically have more time to react by walking backwards while shooting at them, but the only way you could maintain that is if you weren’t holding the laser sight button; however, at that point, your shots would be given so much random variation that you’d miss most of your shots, meaning you’d run out of ammo and be forced to use melee attacks sooner. Problem is, melee attacks in this game are awful: there’s a half second delay between when you hit the button and when the attack actually happens, and no enemy has any such delay in their attacks. In other words, no matter how you approach the game, you’ll have to deal with unavoidable damage. I can only assume this was done since the player has a regenerating shield, so sustaining lots of damage at once is the only way the game could be challenging, but now the game is less about skill than endurance.
Level design is super bland (only consisting of walls and destructible covers, with enemy placement always just being clusters spread throughout the level), so the game has to rely on enemy variety. Problem is that there isn’t much of that, either. First world just has the standard soldiers, with the only difference being what gun they have equipped (and there isn’t a whole lot of variation on that front, either: normal shots, spread shots, rapid fire…I think that’s it). Second world introduces dogs, which are basically the same except they have a charging melee attack instead of a ranged one (and there’s no delay, unlike your own melee attack). There are also shield enemies, which are the same as normal soldiers except you need to keep the pressure on them or else their shield will regenerate (something that’s kinda hard when your clip runs out every 5 seconds no matter what gun you have, forcing you to reload each time). The third world introduces insects, which are the most annoying enemy since half the time, they’re in the air where you can’t hit them and they can’t hurt you; when they land, they fire a series of quick, guided shots which have all the problems mentioned in the previous paragraph (even if you sidestep out of the way, they can make a sharp turn and still hit you). Fourth world gives some of the soldiers laser-sights and rocket launchers, but if you die here, chances are it’ll be because the previous worlds ate all your ammo and you’re forced to use those awful melee attacks against enemies who don’t convey their own attacks.
After this is the final boss, who attacks by firing rapid bullets in random trajectories and throwing a few rockets, so the only way to avoid damage is to hide behind cover. Then it just stands there, waiting for you to counterattack. At certain points in its health bar, a shield tower will activate and spawn some regular soldiers around it, and once all the shield towers are destroyed, the boss will start to walk around, but it still has the same pattern of “attack, wait, repeat,” still making you hide behind cover until the attacks are done.
By the way, even though the level design is identical each time you play, the game still tries to be like a roguelike. Dying once sends you back to the beginning unless you have enough money to buy the option to start at a later world. Groups of enemies will have their placements swapped, but there will always only be one enemy type per group. There’s a shop between each world, but the three items on offer are randomized each time (and given the frequency of weapon/item drops in-level, only relics are worth buying…and maybe ammo if it ever shows up). I gave hard mode a shot, but it didn’t seem any different from normal mode (maybe the enemies are a bit more aggressive? Maybe enemy clusters are slightly larger?). I even gave endless mode a shot, and it still seems like the same levels as in the campaign; the only difference I noticed is that 1) you’re stuck with the character’s default weapon (enemies don’t drop guns), and 2) instead of relics, you can find a teleporter that sends you to a purple level with new skeleton enemies that explode into smaller enemies when they die.