Here is the full list of all 25 Rikki & Vikki achievements.
Hit a Fiddler with a box while it's vulnerable.
Smash a Picky.
Refuse Dut's offer to enroll in Direct Dutposit.
Pick up or catch 100 boxes during a playthrough.
Smash a Scarab.
Lure a DumBun or BigBun onto a box bridge, then get them to fall.
Warp back to the Rock Cavern from Dut's Discount Warps.
Complete the Irregular Cavern in Single Player Mode.
Bang Rikki or Vikki's head on the ceiling 256 times in one playthrough.
Max out the Irregular Cavern's timer (99:59:59).
Warp to the Irregular Cavern.
Get to Dut's Discount Warps.
Listen to Dut's introduction.
Skip the Intro Sequence.
Watch the entire Intro Sequence without skipping any text.
Complete Single Player Mode without any of Dut's services.
Complete Two Player Mode without any of Dut's services.
Refuse Misery's offer to refight all his guardians.
Complete the Irregular Cavern in Two Player Mode.
Complete Two Player Mode without enrolling in Direct Dutposit and only one player purchasing Credits from Dut.
Complete the Single Player Guardian Refights.
Complete the Two Player Guardian Refights.
Enroll in Direct Dutposit from Level R0.
Get a Fiddler to dunk a box on top of a solid tile, forcing it to break.
Use the Warp Bubbles 100 times during a playthrough.