Welcome to SteamAH, Here is a Guide to every achievement you can get in this game (Including the hidden ones).
Flowers For Diane: Complete The “Flowers For Diana” Mission.
Hitman’s Potty Guard: Complete The “Hitman’s Potty Guard” Mission.
If You Can Dodge A Nugget: Complete The “If You Can Dodge A Nugget” Mission (Need to complete the “A Tale of Two Janitors” and “The Hitman’s Potty Guard’ missions before you can complete this one).
Opposite’s Attract: Complete The “Opposite’s Attract” Mission (Need to complete the “A Tale of Two Janitors” and “Flowers For Diana” missions before you can complete this one).
Cain’s not Able: Complete The “Cain’s not Able” Mission (Need to complete the “Flowers For Diana” and “The Hitman’s Potty Guard” missions before you can complete this one).
Things That Go Boom: Complete The “Things That Go Boom” Mission (Need to complete the “Flowers For Diana,” “Opposites Attract” and “If You Can Dodge A Nugget” missions to complete this one).
Breaking Sad: Complete The “Breaking Sad” Mission (Need to complete the “If You Can Dodge A Nugget” and “Cain’s Not Able” missions to complete this one).
Creature Feature: Complete The “Creature Feature” Mission (Need to complete the “If You Can Dodge A Nugget,” “Things That Go Boom” and “Breaking Sad” missions to complete this one).
If you need help with completing these missions, then feel free to look at this guide here: Kindergarten 2 Walkthrough
You can also find how to get each and every monstermon card there so this should help out.
Hard Knock Life: Get Killed For Your Very First Time.
What To Wear?: You Can Unlock This By Killing A Certain Character, Getting Their Outfit, And Changing Once You Get Back Home. (p.s. you do not need to finish the day once you’ve unlocked their outfit)
Starting Your Monstermon Journey: Collect Your First Monstermon Card. (Find A Monstermon Card Anywhere In The School)
Ready To Battle!: Collect 10 Monstermon Cards And Select Any 10 Of Them To Make A Full Deck. (p.s. From This Point Forward You Can Unlock The Mode Where You Battle Other Characters With Your Monstermon Cards To Unlock More Monstermon Cards.
Pewter City: Challenge Any Character (Except The Teachers) To A Monstermon Battle And Defeat them.
Big League: Win All The Monstermon Battles You Could Have At Recess.
Gotta Get ’em All!: Collect Every Monstermon Card You Could Possibly Find. (If You Need Help, Look At The Guide I Linked Above.)
Fashionista: Collect Every Outfit In The Game. (You’re Gonna Need The Gotta Get em’ All! Achievement Before You Can Unlock This One.)
Cat Collector: Give Away Every Cat Agnes (The Dumpster Lady) Gives You To A Certain New Owner.
To Get Smoky, Go To The Normal Class And Wait It Out Til’ Lunch (Because you need the passbook). During Lunch Grab A Burger, and Wait It Out Til’ Recess. Once It’s Recess, Go To Where The Dumpster Is Located and bang on it to talk to Agnes. Once You Give Her The Burger, She’ll Give You Smoky. Once You Get Him, Head Inside, Go Into The Boys Bathroom And Give Smoky To The Previous Hall Monitor.
To Get Gravy, Follow The “The Hitman’s Potty Guard” Quest Until You Reach Lunchtime. Go Ahead And Grab Vegan Food For Cindy, But Save It Until You Get To Recess. Once You’re At Recess Go To The Dumpster And Talk To Agnes Again. Instead of Giving Her A Burger, Give Her The Salad The Lunch Lady Gave You, And She’ll Give You Gravy. Head Back Out To Recess, And Give Gravy To Cindy.
To Get Tenders, Follow The “If You Can Dodge A Nugget” Quest Until You Get To Recess. Instead Of Giving Nugget The Nuggets, Go And Talk To Agnes Once More. Instead Of Giving Her A Hamburger, Give Her The Nuggets And She’ll Give You Tenders. Once Back To Recess, Talk To Nugget And Give Him Tenders. He Will Hate Your For Not Keeping The Nuggets, But Will Appeciate You Gave Him The Cat Instead.
Once You’ve Done Everything Listed Here, You Should Get The Achievement, As Well As A Few Monstermon Cards 😉
(p.s. If You Still Want To Microwave The Cat You Are Now Classified As A Horrible Human Being)