Here are my tips that helped me get to at least level 21 level 41

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Tip 1: Run and gun-

Try to run along the perimeter in one direction only, like its a big square track. So stay away from the middle as much as possible. When you don't have all the weapons unlocked [levels 1-15], try to kill only whats in front of you as much as possible (but its always good to deviate to kill moltov cocktail throwers/ ***gold chain wearing thieves***). Running backwards to get power ups is a fools errand and leads to death often. Don't do it unless you want to risk getting mobbed or are under the influence of a blue can to get in and get out. If you pew pew the zombies in front of your run path, it's easier to collect their loot drops without getting bum rushed. I keep running the full perimeter pattern trying to reduce zombies until I get to the single digits at the end of wave 3 of 3 (the wave and remaining zombies # in the wave can be tracked in the top right). Getting too close to the border wall will open you up to higher chances of getting caught in missed moltov cocktail blasts that hit the wall next to you. There seems to be one benefit to hugging the wall. At level 30 and higher Bosses will less likely be infront of your run path, so tapping the wall when you are behind cover may help keep the trailing bosses in line.


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Tip 2. Order a box to go -

You can use boxes in the early levels (up to level 14?) in the middle of the thin walk ways. If placed correctly it will allow you to get a little bit of breathing room when you run into that area from the first kind of boss. This tip stopped working for me when the "they/them" 2nd boss type zombies spawn (I am not a biologist, so I am unable to call those bulky strong blondes 'females', sorry) . The auto turret works like a box [that shoots ^_^] as an obstacle. turrets can help clean up bosses on wave 3 of 3 but they don't last forever and have a long cool down to reload. As of 5/7/2022 you may want to deploy them with a double click since you wont drop them immediately on the first click. The second type of box will charge any safe place you try to make if the ai detects you blocked off all paths, every time.
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Tip 3. Know your guns-

The standard rifle works fine until you get the traditional weapon of choice against zombies...the classic shot gun. Always reload by pressing "R" when the action wanes so you can be fully loaded the next time things get sporty. The best gun is the mini-gun [ which spawns on level 15]. Just keep its barrel cycling always. So practice tap firing every few steps even if nothing is there. Tap firing in front of your run path helps reduce the mob ahead of you that is off screen when they would normally slow your escape route if they mob up close to you. Tap burst will allows the mini gun to not over heat but be ready for combat. You have to make sure the heat doesn't max out so keep an eye on it. This will help you push through a wall of zombies with less chances of getting stuck in the mob. use the number key to scroll to your weapon at the start of the round for best results. Switching to the sniper rifle is useful for taking out bosses if you are at the end of wave 3 of 3 and want to kill the bosses quicker.

quick reference of the best safe crowd clearing main weapons: ar<shotty< mini gun.
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Tip 4: Try to not kill boss zombies in tight walkways-

It would be unfortunate to find a fat whale of a zombie blocking your escape path down the line. If this does happens, don't despair for they can be removed with a rocket launcher.
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Tip 5. Wood chipper the bosses last-

They have alot of health. Beating a round is less about killing and more about not being killed. You don't need to deal with the bosses immediately to win. leaving bosses for last works well until level 20. On level 20 there is 5, so killing one if an opportunity is available is helpful because you are less likely to run into one in front of you as you run along the perimeter. If you blow up the corpses with the rocket launcher they normally spawn an item
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Tip 6. Moar Easier Mode, Please-

You can adjust the map level by clicking the yellow tabs on the left side after you get passed the first main menu, but before you resume your campaign if you want to play a more relaxed game.

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Tip 7: Save somefin' for the commiequeens-

Throwing grenades at bosses stuns bosses, giving you a way to put some distance from the 2nd kind of boss (who can gain on you if you tap fire the mini gun unskillfully). So saving them for boss escapes helps in later levels. I reccommend to throw a grenade anytime a boss appears anywhere that isn't from where you are running from since it takes time to deteonate. By thowing a grenade on sight reflexively when you see one pop up where they shouldn't(example in front of you), it can potentially save your level run because if you double back and have to mow an escape path you will get slowed. If the boss happens to get hit by the hail mary pray grenade it may by you the half a second needed to get away where you otherwise wouldn't. Most of the time you're dead regardless, but this has helped me around 40% of the time which adds up to saving alot level attempts
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Tip 8 Disarm the bomb

Shoot the explosive cans after you run passed them. It's basically like disabling a land mine. Think of it as a gift for future you. Make sure you aren't in the blast radius. Blowing them up proactively prevents you from blowing them up on accident if you get to close to them when your getting swarmed/ low on life/ or blinded.
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Tip 9: ( a tip from publicly posted comment from zyxophoj, but I haven't vetted this personally yet)
"Chainsaw: emergency weapon. When you have to charge through a horde of trash to escape the claws of a landwhale, this won't let you down."

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Tip 10:You cannot suicide after the round ends. so all explosives are safe when the last zombie falls.
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Tip 11: f a thief steals you gear AFTER level 20, and you are unable to get your stuff back but the round is nearly won: finish the round. Your gear will respawn when you go up a level, so you WON'T be handicapped.
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*new* Tip 12: where your mouse cursor to our character changes the field of view. so if its closer to the self defender your character will be more in the middle of the screen. the farther the cursor the more you character will be closer to the edge of your screen
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*new* tip 13: the new wave will spawn when you drop the current number of braindead to ~20-30. more zombies spawn at once including a higher chance for a both to spawn in your run path. so you can be slightly better prepared


Good luck. The salt 'MUST' flow.