SRAM 11 speed Apex derailleur w/ SRAM GX 10-42t cassette
I think you have two options:


According to Zinn, you can use an Exact Actuation (not X-actuation) Rear Derailleur with the shifter, but I don't think there are any that take such a big cassette. So, you'd probably have to get a new cassette too. 
Use a JTEK Shiftmate 9. A Shiftmate is basically a pulley that you shove into the derailleur or shifter which adjusts the cable pull between two systems. The Shiftmate 9 takes a SRAM 11 speed road shifter and a SRAM 11 speed mountain derailleur and a Shimano/SRAM/Campagnolo cassette and makes them work together when you hook up the Shiftmate into the derailleur. Note that this is likely going to be a special order item made by a small company in the UK, and not cheap (looks to be ~41 USD on SJS Cycles; there are other retailers world wide, though).