The majority(18-22 out of the 26) of these achievements are extremely easy to get quickly, and can be done so even before you can get any card drops(under 2hrs)!
I will explain how I did so in this (hopefully well-laid out) helpful guide!
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#1 "Thank You!"----You bought the game, and will recieve this achievement when you first run the game. Easiest one to get!


#2 "Test Achievement"----Just press the "U" key on your keyboard at the main menu to "earn" this achievement.


#3 "IT'S PRETTY MUCH LIKE NORMAL."---- Simply go into the Custom Levels, and play the "New" level.



#4/5 "Ma' own place!" and "Now you're doin' it right!"---- Go into 'Custom Levels', choose 'Create/Edit' and then choose 'New Level'. Name it, pick a background color, and a song. Just draw a dot then go to the edit menu and click on the Save Icon(white disk) in the upper left-hand corner. After you get an achievement, go back into the edit menu again but this time click the Delete(red trash can) icon in the upper right-hand corner. You will then get another achievement.



#6/7 "You have patience!" and "Played against the machine"---- Choose one human player and a few AI players, but WAIT at the map selection screen and simply let it sit there until a TIP pops up, giving you the achievement), THEN start a match, and play rounds until you get the achievement.


#8. "MENS VS MACHINES"---- Choose 4 AI and 4 Human players, start a match, and play rounds until you get the achievement.


#9 "All skill!"---- Start a match with 2 human players with NO powerups and select the map Pagoda.


#10. "Winding up!"---- Choose 2 humans,@ smiling s no AI, and have Player 2 jump right next to you, then as Player 1 just jump straight up 3 times and land that hit for the achievement!

#11/12 "So coordinated! #1" and "So coordinated! #2"---- Simply start up a match with no AI and 4 human players, and press the jump key for all 4 players at the same time, then do the same thing with the punch key. I personally found these two easier to do by myself with a controller plugged in for one player.


#13/14 "A Match Made in Heaven! #1" "BLAAAAAAAAAAARRRRRRPPPPPP"---- Just start a match with Free for All and Splorch active, and select the map Duel.


#15 "A Match Made in Heaven! #2"---- You will have to unlock the map Space first to earn this one, you may find it easier to follow my instructions for #17-22 first to unlock it. Once you do have Space unlocked, just start a Free for All match with Sumo Steam Guns active on the map Space.


#16 "Better pay attention"---- Set up a Free for All match with 2 Round Shuffle active, and go into advanced to turn Super Power Ups ON and Respawn ON as well, then simply start on any map.

For the next chunk of achievements(#17-22) I set a match up with the following settings: Free for All, No Modifier, Splorch Item Spawn, 7 AI, Never reshuffle. Under Advanced settings, I used: Super Power Up-ON, Auto Respawn-OFF, Comp. Mode- OFF, and Special Round-ON.
Then I started the match and let it idle for quite some time, I had earned the following achievements this way when I came back to it AND also played for a little while.
I've also included descriptions on how to earn them individually as well:

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------#17 Found it!---- While playing with power ups ON, get the item drop that looks like the icon in this achievement.
#18 He really is super..---- While playing, keep getting the character costume power up until you change into the Superman costume.
#19 Told you they were fast---- Finish a round in under 5 sec., another easy way to get this is to play with just 2 players, and have one jump off the map.
#20 It actually happens pretty often..---- Play with AI and live till the round ends without hitting attack
#21 BOOT SQUID!---- Get boot hat after(or maybe before) becoming a squid.(Thanks go to @ smiling senshi!
#22 How it's meant to be played!---- Play 100 rounds w/o exiting to the menu.


#23 "Unlock Mexico"---- Play/idle a LOT of matches to unlock this map. I'm not sure of the exact number, probly 500+.


#24 "Man, that thing's cute"---- Unnlock Mexico, when you're at the map select screen go past Mexico to what looks like a blank map. Pick it, it, start the match, and a lizard-thing should pop up.


#25 "This should be the default"---- I started a match with no AI and 2 human players and Item Respawn on FAST. I grabbed the character costume power up with both of players until I got them to be squids. I tried this with 1 Human+1 AI, but even when I became a squid it wouldn't give it to me.

****NOTE: #26 HAS BEEN REMOVED FROM STEAM'S 'SUMO REVISE' ACHIEVEMENT LIST****
#26 "Ultimate PARRRRRRTY!"---- Okay, this SEEMS to be easy if you have 4 controllers and a kb+m, but even with all 8 human players being signed in it still will not give it to me(even hours after idling with Super Splorch on).
I've tried everything and I don't understand why this one doesn't work for me!
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Hopefully this guide has helped you out!
If you like, feel free to leave a comment/thumbs up/rating, but hey...you don't have to.
Thanks!, -+- E.F.