10 random words from a really large dictionary. Narrow down to, say words three to five letters, and 128 bits get tricky. It's easier in the sense that we have a certain active/passive vocabulary - but you'd probably need a mnemonic with words too.
[ed: input is definitely a challenge. I've considered making various alternative input methods - but in the end 128 bits is a lot of data to input without error. Especially on a phone. Of course on a phone you might have a pin protected secure enclave for example, or use an external nfc key etc.
But it then we're back to some way to remember 128 bits - and maybe even without the luxury of "practice" by way of typing it a few times a day...]
