The game has 3 modes the player can choose from which has different kinds of mechanics and amount of levels.
Campaign[edit]
Campaign is a mode with up to 73 Levels, the goal in this game mode is get all 5 ship parts and get out of the planet after, along the way you'll get extra primary weapons, secondary weapons, and you'll be able to level up your character as well to upgrade its stats, these stats are HP, Armor, Reload, Ammo, and Experience Gain, every time you successfully level up you get 5 points in which you can spend it to improve your stats to your own liking.
Campaign so far is the easiest mode of the 3, it is recommended to beginners due to the level up system and the game isn't too harsh with level requirements for the first 30 - 40 Levels, in this game mode you would want to get as much Experience Gain as possible to get to higher levels quicker, this also means you should kill as much enemies as you can in each level until almost nothing is left as long as you have extra ammo and everything is manageable, the rest will be mentioned in an another page here, but one thing I could suggest is that some power-ups could hinder your monster farming, and here they are: Chrono Moderator, Accelerator (this could ruin your positioning), Panic Emitter, and Freezer to some extent, once you gain access to AOE weapons such as the Rocket Launcher you would want to kill groups of enemies at once while also saving your ammo, and the mentioned power-ups could hinder that because unlike Firing Range, your character can be levelled up which means you aren't that easy to kill as long as you know how to efficiently use your Primary Weapons and you know how to keep your distance, you won't even use your Secondary Weapons for most of the time unless your life is threatened or enemy hordes become extremely difficult to manage (which rarely happens), tractors could also become somewhat useless here due to the reason mentioned above, so you also won't likely to use them as well, you will enjoy the game well here and this is a good start, saving ammo in Campaign is recommended especially for weapons like the Sonic Cannon, in other words once you gain access to these weapons do not use them immediately, stock up on ammo first.
Campaign will pit you out on levels with different kinds of finishing requirements and mechanics, some levels will require you to kill a certain amount of enemies, some will require you to get or use an important item (mostly Replacement Item levels) before you can leave, some requires you to activate a switch or set of switches to unlock various gates or forcefields, some requires you to hold on for a certain amount of time before you can leave and finally some levels could limit your vision too and it will return to normal when you meet the requirements, sometimes not at all, some would require you to kill mutant leaders or destroy spawners marked with a blue circle.
Firing Range[edit]
Firing Range is an another mode with up to 40 Levels, the goal in this mode unlike Campaign is simply survive and get out of the level as soon as you meet the requirements to be able to get out, this is because you cannot upgrade your character so this is more skill and luck reliant than ever, this could be considered a practice mode for you or a way to improve your skills even further in this game because Firing Range is even harder than Campaign.
Firing Range is a reverse to how Campaign works, due to no level system this means your character will remain in base stats all the way to the last Firing Range level, this makes the difficulty of this gamemode go even higher and this time you will need all the help you can get, the finishing requirements for this game mode could go all the way up to more than 200 kills quickly so you would want to get out as soon as possible when you meet the requirements, Mission 11 is also unique in such a way that if you don't get to the finish line immediately after it appears, the green circle will disappear and you'll be stuck until you restart, Mission 19 is also unique because as soon as you reach the kill requirement the level is already over, all Power-ups here especially the ones supposedly hinder you in Campaign all gain increased value here, this also makes the tractor more useful as there isn't anything much to defend yourself, the skill that you need here the most is proper weapon usage, ammo reservation, and keeping your distance from mutants especially dangerous ones at all times, a single explosion from a Detonator Mutant will quickly strip off your entire Armor bar for example, you will also need some luck on your side, some levels in Firing Range also love to abuse the power of Monster Spawners so if you notice that the enemy hordes for a level is somehow way too thick, restart the level and walk around to check out for spawners and when you do see them, destroy it immediately with a high damage weapon and especially with a Plasmagun, saving your ammo is an absolute must for this game mode as the hordes in this gamemode are even thicker compared to Campaign, which means if an ammo drops on an enemy horde you cannot risk to dive and get it because you will most likely die quick.
Most levels in Firing Range require you to kill a certain number of mutants, and this is often high so you have to be really careful, there are a few levels (3 - 4 at most) that requires activating switches but they are not as complex as what Campaign has, but what makes it hard anyway is the high kill count requirement, these are the only finishing requirements Firing Range has.
Survival[edit]
Survival is the last mode this game has, and the rules for Survival is simple, just simply survive for as long as you can and get as much score as you can too, each Survival levels are different, they differ in what weapons they provide and what mutants and extra enemies it contains, like Survival 7 for example provides a Machinegun and Shocker, and the mutants it contains are the Spider Mutant and Humanoid Axe Mutant, Survival 7 is also dark so your vision is limited.
There isn't anything much to say about Survival mode, this is purely skill reliant unlike Firing Range due to how its levels work in such a way that you won't fight hordes of enemies immediately as soon as your start, the longer you last in a Survival level the harder it gets, you must reserve your ammo and learn how to deal with enemy hordes well to increase your survival chances.