Have an orb bounce only between slimed pegs 6 times before touching anything else
Unlocks: Spinventor
With enough luck in getting the right orbs, this is relatively easy to achieve, and comparably simpler than unlocking both other characters. These are the orbs and relics that help with this achievement:
Orbs: Splatorb, Twooze Company, Chemiball Hazard, Generatorb, Ballm; Multiball orbs
Relics: Prime Slime, Gift That Keeps On Giving
A major reason why this is easy to achieve is because this achievement can be completed in the Post-battle screen. Also, any orb that applies slime to pegs works for this, not just Splatorb as you might initially assume. So all you need to do is get one or 2 orbs that apply slime to pegs and try to get a refresh on the same turn as the battle ends, as this will bring back a few of the previously slimed pegs and make it easier to get 6 bounces in the Post-battle screen.
As I said, there are many orbs that apply slime, but Splatorb is arguably the best to get as the slime it applies makes the pegs durable. For this same reason, the Gift relic also helps here. The new Prime Slime relic helps tremendously as well, as it makes it so any orb can apply slime. If you get this relic, aim for Matryorbshka or Circcae for maximum slime generation.
information from steam guide By Blinky called "Peglin All Achievements Guide!" full credit by him for reporting this.