To begin, you will need to find a birthday table with big present on 2nd level of the game, this can be done on any difficulty and it can spawn on either side

After building a wall past the table, you will be able to activate it, but you need guests before starting the party... Wait until 2-3 unemployed kids join the celebration. This will allow you to pay at the table again, revealing the monster truck

After hopping in a truck, you simple need to "sprint" for a few seconds to get tired a bit. Afterwards stand still, you will slap the truck and get the achievement

Although missable at first, the achievement is easily unlocked once you know what you're doing. Hope this guide helped and best of luck building your kingdom!