Monster Jam Steel Titans 2

To get this achievement you need all the default paint jobs for all 38 trucks.

Starters you unlock from the beginning. So it's a gimme.

Unlocking 20 of the trucks for the different teams come by beating the World Career events. 5 are unlocked once you get all the Collectables in each team's Area. 5 for Wildcards and 5 for Ruff Crowd. 10 for Defenders, Demolition Crew and Undead. None for starters or Crazy Creatures. Here's a voice free guide that the creator set up in chapters so you can click through on the website to see what you're missing and how to get them.
To unlock the 6th vehicle in each class you need to get the entire Team XP to 25000. Quick way to level up once you're closer to the end of the game is that in the World Finals 19 or 20 , You get 1000-1200 XP per finish in first and you can replay the event over and over to keep leveling up. The final event gets about 3000-3400 XP but overall takes longer to complete as it's a lap race or 2 min freestyle. Doing the 2 wheel event you can get a good wheelie, mix with backflip and get a 10.0 score. Start another stunt and then hold B to cancel and the event ends in about 20-40 seconds. X to retry and repeat as wanted.

For the Crazy Creatures there are 2 ways to get them. You unlock one for each team of the previous 5 by getting 2 vehicles of each team to Max Level.

Crushtation from WIldcards
Octon8er from Ruff Crowd
Jurassic Attack from Defenders
Dragon from Demolition Crew
Megalodon from Undead

Or the secondary secret way is to follow the clues next to the garage icon for each location. Here's a video how to do that and also get each of the truck's world challenges.
Then the final one Sparkle Smash comes from getting one of the previous ones to level 25.

To unlock this achievement, you must complete the following other achievements: