The simplest is probably to make a deal with criminals that have an existing money laundering network. The trick would be avoiding having said criminals simply

The simplest is probably to make a deal with criminals that have an existing money laundering network. The trick would be avoiding having said criminals simply kidnapping you and forcing you to reveal the keys. Low volume ways would be to use the bitcoin to purchase drugs or other illegal goods by the kilo and sell the drugs the usual way. It's unlikely that the person needs to full billion immediately. Five or ten million a year could be enough for many people and at that kind of level exchanging for monero could work. There is also the option of sending small amounts to thousands of real people's bitcoin wallets (and lots more to wallets you control) and letting those people deal with the FBI first.