The Binding of Isaac: Rebirth

This achievement unlocked by unlocking the Cat Got Your Tongue Challenge. To do so you must collect three different 'Guppy' items within the same run transforming you into Guppy.

Items that will transform you into Guppy are -

Guppys Head
Guppys Tail
Guppys Paw
Dead Cat

The item Guppys Hairball will also contribute to the transformation but this wont be available until you unlock it with your first transformation.

The best methods for becoming Guppy are first to make sure you visit all Curse Rooms you come across, Red Chests have a chance of holding Guppy items inside them. Secondly try to get Deals with the Devil as often as possible as Guppy items are possible deal offerings.

To get a Deal with the Devil you need to try and fill certain criteria to make the room appear after defeating the boss on any given floor. Here is a list of criteria and the probability increase they give for finding a room.

Take no Red Heart damage for the floor - +99% (You can take Spirit Heart damage)
Take no Red Heart damage against the boss - +35%
Holding the Pentagram - +20% (+10% with Afterbirth)
Holding a second Pentagram - +10% (+5% with Afterbirth)
Holding Black Candle - +30% (+15% with Afterbirth)
Holding Book of Revelations - +35% (+17.5% with Afterbirth)
Killing a Judgement Beggar with a bomb - +35%
Blowing up Shopkeeper - +10%
Holding Book of Belial - +2500% (+12.5% with Afterbirth)
Holding Goat Head - Guarantees Devil Room on every possible floor

Finally if you have visited a Devil Room in the past the following multipliers will be applied to your current % chance of finding another.

Devil Room found within last two floors - x0.5
Devil Room found more than two floors ago - x0.25

This means that if you have found one within the last two floors your chances of finding another for the two floors following are cut in half, any floors beyond that it goes down by 25%