HELPFUL HINTS FOR PEOPLE WHO FIND THIS GAME HARD:
This game is largely based on skill. With little lag & relatively solid controls, winning is dependent largely on the player's skill, and not just about having the biggest and baddest guns.
#1: FINISH ALL OBJECTIVES IN ONE ROUND AS FAST A POSSIBLE. I cannot stress this enough. Doing so gets you early access to better guns, which is crucial in completing the game.
#2: When you start the game, explore methodically. Try to memorize which sections of the stage you already visited. I recommend you search in order, like left-to-right or top-to-bottom. This saves time from going back to the same place.
#3:Time is your biggest enemy. Look at the time frequently while playing. Make sure you finish all objectives before time runs out; often, the thing that kills me at the end of reach round isn't HP, but time itself.
#4: Don't get the health box as soon as you find it. Get it only when you need it (like 50hp or less), because there's a limited amount of health boxes.
#5: Don't panic when you're surrounded by enemies. These enemies tend to wait for about 0.5 seconds before shooting, so calm down & aim for their heads leisurely. Or, if possible, shoot & jump behind platforms to avoid getting shot back.
#6: The best guns to use are the Gauss Rifle, Gauss Pistol, M3-556 Minigun, SS-500, AP-1, Model 5-454, and the XR-556. Try to equip at least 1 of these guns in any game; they kill enemies quickly and effectively.
#7: Do NOT use the Artifact 23. It's the last weapon you unlock, but it SUCKS. It fires a slow purple orb that disintegrates any enemy it touches, but its firing rate & speed are too slow. You're better off with the XR-556, which you can get in around early-mid game.
#8: If you have a decent gun like the XR-556 (or any of the guns I recommended), try getting the kill objectives by standing in single, straight, and open area. Kill anyone that walks from left or right. This racks up kills & scores quickly.
#9: Shotguns are very good when you're moving around (they kill enemies usually in 1 hit). But, they're bad when you're staying in one place, since enemies can gang up on you & hit you while you're reloading or waiting for the gun's cooldown after it shoots.
#10: The Acid Launcher is the SECOND WORST weapon to use. You have to literally hit an enemy with a slow acid glob, and even then, you have to keep hitting it with the blobs or wait forever for the acid to kill the guy. It's too inefficient.
#11: On the last mission, don't be fooled by the name of the gun "hyper death blaster". You have to kill 5 enemies with it in an objective, but it's not really a gun. It shoots little yellow pellets that do almost no damage. NEVER use this gun; it's hands down THE WORST weapon to use (you need 200+ rounds to kill one enemy).
#12: The controls are sensitive depending on how long you press the buttons; you jump higher if you press down on the jump button longer, and you move very slightly if you lightly tap the buttons for moving left and right.
#13: The M35 Rocket Launcher has 2 rounds & needs about 1.5 seconds to reload. It's okay as a gun, but it's best attribute is being able to DAMAGE ENEMIES BEHIND WALLS. That's right; with this weapon, you can walk up to any enemy behind an obstacle & still kill him without getting shot back.