* Oxymoron: This is the most prominent device. "Monstrous" and "little" are contradictory terms, creating an unexpected and striking image. This juxtaposition emphasizes the speaker's paradoxical nature - powerful yet diminutive, or menacing yet seemingly harmless.
* Metaphor: While not explicitly stated, "voice" can be interpreted as a metaphor for the speaker's power or influence. The "monstrous little voice" might represent a seemingly insignificant force that holds immense power, or a powerful voice that is being used for something small or trivial.
* Alliteration: The repetition of the "l" sound in "I'll speak a monstrous little voice" adds a rhythmic quality and emphasizes the speaker's determination.
The combination of these devices creates a sense of intrigue and ambiguity, leaving the reader to ponder the speaker's true nature and intentions.