• The game only counts the highest modifier that you earned for each kill. Most of them do not stack. +1 for hat trick is a notable exception. If you get +5 for quickdraw, it won't give you anything else if it is also a chain kill, consecutive headshot, and power shot.
  
• Combo points (white numbers) are multiplied with the damage (yellow numbers) you've done recently to calculate your score. Whenever this guide mentions a +1 to +10 bonus, it is referring to combo points. The longer you can maintain a combo, the bigger those numbers get before the score is tallied. Making both of these numbers as high as possible is critical to getting a high score in any of the game modes. Rapid fire weapons and explosives tend to have high damage, but low combos. Penetrating weapons tend to have low overall damage, but higher combos, thanks to power shots.
  
• The amount of time you have before your combo flashes away is variable. If you just kill one enemy, you have about one second to score more points. If you have 500 damage and a +100 combo modifier, then you have more like 5 seconds. Taking damage or killing a hostage ends your combo prematurely.
  
• Right click on the mouse to jump. This may sound stupidly obvious, but I had 46 hours play time and was 96% of the way through the 50,000 kills achievement before I realized this. I had been using the space bar the whole time. You can only jump while running and I had only tried right clicking when stationary and behind cover.
  
• You can switch between any of your available weapons during a mission through the armory in the pause menu. It is situationally useful, but it can become tedious to do all the time. It is possible to get 3 stars on all of the missions without needing to do this.
  
• When an enemy is preparing to shoot, a red reticule will appear around them. If you kill them before they fire a projectile, you get a +5 quickdraw bonus. Helmeted enemies will fire red projectiles much more frequently than other enemies.
  
• Sniper rifle, magnum, auto sniper, and light machine gun all have the ability to penetrate obstacles, giving a +5 power shot bonus if you kill an enemy through a single layer. This is like permanently having the super bullets powerup. If you shoot through two or more obstacles, you get a +10 ultra power shot bonus instead. Getting a headshot on a helmeted enemy with one of these weapons gives you +1 for hat trick and +5 power shot for shooting through the helmet. A penetrating headshot on a helmeted enemy through a wall gives the +10 bonus.
  
• Killing 3 or more enemies without pausing long enough to reload gives +3 chain kill bonus. Getting more than one headshot in a row only gives +2 bonus. When using one of the penetrating weapons you will get more points by aiming for the torso and getting one kill per second until your clip is empty. Getting 10 consecutive headshots is more difficult and will only give (9*2) = +18 bonus, whereas getting 10 kills with one clip will give (8*3) = +24 bonus. In this example, you should only aim for the head if the enemy has a helmet on, as explained in the previous tip.
  
• The plasma rifle and the 4 penetrating weapons all hit their target instantaneously, making it easier to hit targets that are only briefly onscreen, like jets and red ninjas. All other weapons have slower moving projectiles.
  
• Hostages frequently leave an armor pickup behind if you are damaged when you rescue them. They leave 10 coins about 95% of the time if you have full armor when you rescue them. The other 5% of the time they leave a random powerup. If you don't click on it before it flashes and disappears you lose the powerup. However, if the pickup scrolls off the screen due to level progress it will automatically be applied to your character. If you suddenly have hero time or robocop armor and you have no idea why, this is usually what happened.
  
• Destructible crates also have a small chance of containing a powerup. Most of the time they have 3 gold.

• The powerups can be quickly activated with the 1,2,3, 4 keys. You can also click on the icons in the lower left corner.
  
• Airstrikes can potentially give you a huge boost to your score when used on large groups of enemies. You can cause over 200 points of damage at once, even if the airstrike hits dead bodies. It does not work indoors, for obvious reasons, so you can't use it in the elevator sections of Metropolis or the desert tomb segments.
  
• Power shot increases the damage your weapon does, lets you shoot through walls, and makes you bullet hit the target instantly. Use this if there's lots of enemies behind cover and you don't have a weapon with penetration. This pairs up best with the plasma rifle for maximum damage.
  
• Hero times makes it much easier to shoot incoming bullets out of the way. Try to use it when entering a fresh group of enemies, otherwise you'll waste half of it watching your guy cartwheel to the next section of the level.
  
• All of the powerups work best with fast firing weapons. For example, wearing robocop armor makes you invulnerable and automatically gives you a +2 deflection bonus if you're out of cover and get shot. Having a rapid fire weapon means you're out of cover more often to take advantage of this.

• Don't bother trying to go for stars until you have several weapons unlocked. The pistol isn't good enough to get more than 1 star on most missions.
  
• The trick for getting 3 stars on most levels is to use either an automatic explosive weapon (quadzooka or auto launcher), or a weapon with a very high rate of fire like the plasma rifle (highly recommended) or minigun. Focus fire on each enemy and shoot the dead body until it flashes away before moving onto the next one. This lets you build up a larger bonus. Also, many levels have chickens that can be used to maintain your combo in between segments of a level.
  
• Best mission for jets by far is Tropics 6. There's 2 jets at the beginning and 4 more jets towards the end.
  
• The mission with the most tanks is Tropics 15, which has 2.
  
• Timebomb mode will also frequently have several tanks, but is less consistant.
  
• Getting the achievements for hostages is easiest when playing the first scene of Desert 10. Restart once you've rescued the first two and do it again as needed.
  
• Be careful using a shotgun or other inaccurate weapon around hostages. Your combo automatically ends after hitting one and you lose the opportunity for a powerup.
  
• Hostages are curiously immune to explosive damage, so it's safe to hit them with grenade launchers. You can take advantage of this to help you get the "rescue 100 hostages in a row" achievement. A direct hit with a bazooka rocket will still kill them, but the rest of the explosive weapons are fine.
  
• The arcade mode achievements are pretty easy if you have plenty of continues saved up. Just keep going until you get 250,000 points. Arcade mode is also good if you are getting bored of playing missions in the same order, since it picks a random mission from the 3 zones.
  
• The best missions for barrel or chain reaction kills are Metro 7 & 14.
  
• Metro 5 is a good level to farm 2000 grenade kills. They are right at the beginning, making restarting quick, and the enemies are easy to hit. There's 9 agents and 3 chickens offscreen to the right that you can get. Tropics 11 and Desert 9 are also decent candidates if you get sick of doing the same mission over and over, but the targets take a bit longer to kill.
  
• Tropics 12 is a pretty good mission for racking up explosive kills quickly.
  
• Desert 2 & 14 both have densely packed enemies all throughout the level. This makes them good for getting your weapon kills.
  
• The Invincible achievement requires you to play through the game in classic mode from beginning to end in one sitting and without dying. Despite what the steam achievement says, this can't be done in survival mode. When you complete Tropics on survival, it sends you back to the main menu, which resets this achievement's progress. The achievement isn't too difficult, just play cautiously and if you've already taken one or two hits, then be particularly careful to rescue hostages to get armor. Robocop also replenishes your armor once the invulnerability wears off. Buying a new hat at the armory from the pause screen will also replenish your armor if you've taken one hit. Buying a new outfit will restore your armor if you are bare-chested. This only works for new purchases. Equipping another hat that you already owned won't restore your armor. If you like to change your outfit every once in a while like I did, you could wait until you are damaged to get the most use out of it.

In case you need to find a particular level feature for a quest like spikes, jets, or chickens, here's a list of what levels have which gameplay elements. I may have missed a hostage here and there, but this should be most of them.

Tropics
Mission 1: 2 jets, hostage,
Mission 2: jet, hostage, 2 jets,
Mission 3: chickens, 4 hostages
Mission 4: Run N Gun, hostage, 2 jets, hostage,
Mission 5: chickens, 2 hostages, Run N Gun
Mission 6: Run N Gun, 2 jets, hostage, Run N Gun, hostage, 2 red ninjas, 4 jets
Mission 7: chickens, 2 hostages, Run N Gun
Mission 8: hostage, Run N Gun, hostage
Mission 9: 2 jets, hostage, 2 jets
Mission 10: hostage, chain reaction barrels, tank
Mission 11: grenades, Run N Gun, chickens, Run N Gun, grenades,
Mission 12: Run N Gun, hostage, 2 jets, chickens, tank, Run N Gun
Mission 13: Run N Gun, spikes, 2 hostages, chain reaction, Run N Gun, chickens, Run N Gun
Mission 14: grenades, hostage, Run N Gun, chain reaction, tank
Mission 15: 3 hostages, 2 tanks, chain reaction,

Metropolis
Mission 1: chickens, 2 jets, 2 hostages,
Mission 2: 2 hostages, grenades, 2 hostages, chickens, hostage,
Mission 3: 2 hostages,
Mission 4: 2 hostages, Run N Gun, hostage
Mission 5: grenades, 2 hostages, Baron Bear
Mission 6: grenades, 2 hostages,
Mission 7: grenades, many barrels for chain reaction, 2 hostages,
Mission 8: chain reaction, 2 jets, Baron Bear
Mission 9: hostage, 2 jets, riot shields,
Mission 10: riot shields, hostage, Run N Gun, tank,
Mission 11: riot shields, hostage, tank, chickens, hostage, grenades,
Mission 12: riot shields, 2 hostages, Run N Gun, 2 jets, Baron Bear,
Mission 13: chickens, 2 hostages, riot shields, Run N Gun, grenades,
Mission 14: 2 jets, many barrels for chain reaction, tank, Run N Gun,
Mission 15: riot shields, 2 hostages, 2 jets, tank, Baron Bear,

Desert
Mission 1: 2 jets, hostage
Mission 2: 2 hostages, grenades, Run N Gun,
Mission 3: 3 hostages,
Mission 4: 2 hostages, boulder
Mission 5: hostage, grenades, hostage
Mission 6: 3 hostages,
Mission 7: 2 hostages, tank
Mission 8: chickens, hostage, Run N Gun, 2 jets, hostage
Mission 9: grenades, chickens, 2 hostages,
Mission 10: 3 hostages, Run N Gun, tank, jet, best hostage mission
Mission 11: mummies, hostage, boulder, spikes
Mission 12: hostage, mummies, hostage, President
Mission 13: hostage, grenades, hostage, mummies
Mission 14: chickens, Run N Gun, 2 hostages, Run N Gun, 2 jets, hostage
Mission 15: President

I hope this guide was helpful in some way. It started off as me making a list of the level features for myself and I figured I'd write down all the stuff I've observed about the game and make it a guide. If you have any questions or comments, don't hesitate to leave them in the comment section below.