Ok. I still think the sequence is random in the algorithmic information theory sense; i.e., it's incompressible. But I understand you're interested in the adversarial aspect of the scenario.

You only need a halting oracle to compute your adversarial sequence (because that's what it takes to run AIXI). A super-Solomonoff inductor that inducts over all Turing machines with access to halting oracles would be able to learn the sequence, I think. The adversarial sequence for that inductor would require a higher oracle to compute, and so on up the ordinal hierarchy.