Spitlings has 28 achievements, most of which are have rather self-explanatory unlock requirements. I have however also added some additional information for the more involved ones.

First up are achievements awarded for performing certain actions a number of times. These track your progress across rounds and can be progressed and unlocked in any game mode.

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The Great Flood
Spit 10.000 times

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Say it, Don't Spray it
Make 200 attempts to spit with a dry mouth


To increment this one, simply try to shoot after you've already used up all of your ammo.

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Five Second Rule
Slurp up 2000 spits from the ground


Any spits which drop to the ground can be collected to refill your ammo, as long as you're actually missing any. They'll be automatically collected when you touch them, incrementing this achievement.

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Moisturizer
Recharge 500 spits


When you've used up all of your ammo, you can slowly restore it by standing still and pressing "O" (Playstation) or "B" (Xbox).

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Right in Two
Split 5000 bubbles


Bubbles are the bouncing balls you have to destroy in order to beat each level. Hitting one of any size with your spit is what increments this achievement.

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Bubblewrapper
Hit 1500 breakable blocks


For this achievement, "hitting" refers to destroying the blocks with a spit shot.

As with the last set, you'll unlock these achievements by performing certain actions a number of times. You can unlock them in any game mode, with you progress being tracked across rounds.

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Gravity is Not Enough
Hold 'down' while falling for 30 seconds total

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Warm and Squishy
Spend ten minutes total in jelly


This one refers to the non-lethal goop blocks, which let you pass through them while slowing you down.

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Toxic Diver
Dive into the Barrier Jelly five times


Barrier Jelly is found in the hub worlds of story mode, where it blocks of access to the next floor until all levels on the current floor are completed. You can access it at various points, resulting in a respawn.
Note that (as far as I know) all Barrier Jelly will be removed once you complete story mode, meaning you'll have to start a new game to get another shot at this achievement if you didn't get it prior to finishing the game.

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Punctured
Touch 50 spikes

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Disco Lights
Switch the theme in-game 50 times


While in a level, you can use L1/R1 (Playstation) or LB/RB (Xbox) to change its colours to those of any level you've previously beaten. Note that this thus requires you to have beaten at least one level previously.

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Out of Breath
Take the elevator ten times


During story mode, you'll find elevators at the far-right side of each even-numbered floor, right after the drop leading down to the next floor. Simply enter the elevator room and spit at any of the buttons for floors you've already unlocked in order to fast-travel to them

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Stronger Together
Play 90 minutes with at least two players

Next, we have achievements which require certain actions to be performed a single time. As with the previous once, these achievements are unlockable in any game mode.

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Jetpacker
Stay in the air for 15 seconds


This achievement requires you to not touch any floor or enter jelly for 15 seconds. You'll have to spit downwards to keep yourself in the air, spacing out your shots in order to stay air bound for as long as possible.
In order to reach 15 seconds, you'll need a way to refill your ammo along the way. Thus, you should go for this achievement on a level that either contains accessible bubbles to pop for additional ammo or has its floor covered by bounce pads, allowing you to re-collect your spit.
The second level of level pack 09.2 is a good candidate for this.

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Quickstep
Fail a level in under 1 second


There are a couple of levels that start you in the air above spikes, along with some others that start you off very close to hazards.
AN example is the first level of level pack 08.1, where you can quickly propel yourself upwards into ceiling spikes.

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Whiplash
Spend ten seconds caught between two jump pads


You'll have to find a level in which a floor and a ceiling jump pad are facing each other with little distance between them.
The first level of level pack 05.3 is well-suited for this.

The following achievements are exclusive to Spitlings' story mode and will be unlocked automatically as you progress through its levels. Cutscenes are short animated comics which you'll automatically enter after dropping down to certain floors for the first time.

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Mayhem
Watch the first cutscene

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All juiced up
Watch the second cutscene

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Hoisted
Watch the third cutscene

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Total Teamwork
Finish story mode


This achievement will unlock after beating the game's final level, being the only one present on floor 0.

The story mode is split into level packs containing 3 levels each. After completing any of these packs, you'll be able to enter it once again, which will bring you to a showcase room displaying the time you took to complete each level as well as a target time you're meant to beat. You can then enter the levels individually to improve your time where necessary.
Once you've beaten the target time for all three levels in a pack, you'll unlock the associated pack of hard stages, which you can enter through the showcase room. Just like with the regular level packs, you'll have to beat three hard levels and are rewarded a Spitling skin after doing so.
Many of these can be genuinely crushing difficulty-wise - in other words: Good luck!

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Bring It On
Beat 10 B-Side stages

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Hard Mode Master
Beat 20 B-Side stages

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Ultimate Spitter
Beat all B-Side stages

The next section is for all achievements exclusive to the party mode.

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Party On!
Play 10 party sessions

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Experimental Features
Play with a mutator selected


When setting up a party session, the options screen allows you to select one of a number of mutators which alter gameplay in various ways. Simply play a party match with any of these selected.

The game allows you to unlock a bunch of skins for your Spitlings, which you need to collect in order to get the following achievements.

There are 100 skins available in total, 4 of which are unlocked from the start. You'll gain an additional skin for beating each level pack of the story mode, with 36 being available in total. Every pack of B-Side gives you another skin, adding another 36. You'll also automatically receive 4 skins for beating the game, meaning all of these achievements can be completed just by finishing all story mode stages and their B-side variants.
The 20 remaining skins are awarded for completing certain achievements and for playing a number of party mode matches. You'll also be rewarded with a skin for watching through the games credits without skipping them (Either directly after beating the game's final level or through he menus "Extras" section.).

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Free Your Friends
Unlock 30 Spitlings

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Getting Crowded
Unlock 60 Spitlings

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The Whole Town
Unlock 80 Spitlings

I hope I could aid you in completing Spitlings. In case you stumble across any mistakes or poorly worded sections, feel free to leave a comment to tell me.

I'd also very much appreciate if you gave the guide a rating and/or an award in case it was helpful.