Stuck at the same stage really just need 20 more hive kills but it seems pretty difficult and a challenge I'm going to try hardcore first and then switch it up to core if that doesn't go to plan the worst one is probably getting 3 double kill with hive and that's the only tip I can give you cause this is the last one I need too to unlock hero challenged sorry I couldn't help you out

Since both of these specialists are hard to use, I just took my time. For Rejacked, I played nuketown and made sure I died in the other teams spawn.

For the hive, I just played slow. I camped and put my hive in the only entrances. It was a long process, but I eventually got it.

For the HIVE play hardpoint and get your hive rush to the next hardpoint and pre place all hives in doors and ceiling NOT ON FLOOR OR WALL

For the REJACK play FFA or HC TDM use a shotgun with longbarrel wait last second then rejack (works sometimes)! My method it easier have a c4 kill your self rejack and rush spawn so play nuketown uplink its easir just VMP the ***** as they try to leave spawn easy

My last one is the scorestreak combat focus

i completed all of the double kill challenges for the specialists and the one that helps the most is ground war.