This game is relatively easy, and can be played quite casually. However, there are many things you can do to improve your gameplay and be able to get higher scores, and as a result more money, which can help you unlock achievements faster and reach 100% completion.

Keep in mind everything in this guide is strictly my opinion and is not necessarily true.

Here are some tips you can implement when buying upgrades

• Prioritise max health and armor earlier on as it's significantly less expensive than some of the other upgrades. Damage is useful, but you want to focus on surviving.
  
• Max out damage in the order it's displayed. The other weapons are more expensive to upgrade and you should prioritise getting revolver maxed before starting to upgrade your rifle or shotgun.
  
• Do not buy the beam revolver until you have the other two weapons. The beam revolver is strong but it runs out of ammo very quickly and is meant to be used once a round and to shred tanks/bosses.
  
• The best upgrades for each gun assuming you have everything maxed are: Revolver - Beam, Rifle - Heavy/Gattling (mainly preference but I do think heavy is slightly better), Shotgun - Smart
  
• If you have enough to buy either rifle or shotgun but not both, prioritise shotgun. It is significantly better than the rifle, especially with little to no upgrades.
  
• Speed should be upgraded last, as it's important to get yourself more comfortable with the controls before speeding up your movement. It can be tricky to aim while moving at high speeds so it's important that you are comfortable with the controls beforehand.

Here is a tier list of what upgrades I think are best and should be prioritised throughout your runs.

• S - Second Life, Execute
  
• A - Lifesteal, Shockwave, Melee Protection, Armor Steal
  
• B - Electric Bullets, Projectile Protection, Adrenaline Rush
  
• C - Fire Bullets, Heal, Bigger Pockets, Dash
  
• D - Jackpot

My personal favorite combinations of these are "Execute + Fire Bullets", "Second Life + Shockwave", & "Lifesteal + Armor Steal"

If I missed any upgrades or any get added into the future, please let me know in the comments below so I can update the tier list.

Early Game
When starting out in the game, you want to make sure to familiarise yourself with the levels and make sure to use the exploding barrels as they are very helpful earlier on due to your lack of damage.

• You have very little health, so focus on staying alive rather than rushing through
  
• Your melee is capable of one-shotting every enemy besides tanks, use it well. Keep in mind that it has around a ~1 charge-up time before it actually attacks. You're especially vulnerable during that state due to your lack of health and armor, so make sure to position well.

Middle Game
At this point, you should've gone through around 1-2 runs and have a decent grasp on the mechanics and how to make your way around the different levels. There's nothing much more you can do.

• You can afford to rush through the earlier stages due to you having more health and armor at this point, so you only need to begin being cautious once there are 10 or more enemies at once/stronger enemies
  
• Barrels may not be as useful. Use your judgement as to if you would do more damage with your gun or if the barrel would.

Late/End Game
You should have everything maxed out at this point, outside of cosmetics. Therefore, you likely have already beaten the game once. If this is the case, you likely don't need tips, but feel free to read on anyways.

• Avoid using barrels at this point. All your weapons at this stage are significantly better if you shoot the enemies directly.
  
• Make sure you're using the beam revolver and saving it for tanks or when it becomes overwhelming. Do not use it for small waves, use the shotgun for that.
  
• Your primary weapon should be the shotgun, and your rifle should be a secondary choice. The shotgun does way more damage and is just way better for general usage.
  

If you decide to follow these tips, then you surely will breeze through the gameplay in no time.
Remember that these are just based on my opinion and not necessarily factual.

Please note that this is just to be used to help you, and you shouldn't completely dictate your choices in-game based on this guide. The main goal is for you to enjoy the game, and it is best for you to try and figure out the game for yourself. But, if you find yourself struggling, feel free to return to this guide whenever you find yourself needing help.