i did this with friends by doing the earth strike mission "the devils lair". there is a walker before u fight the final boss. u can kill the walker and then return to orbit and redo the mission until u complete the bounty. also, as a heads up, only the player that gets the killing shot recieves credit, but it is also passed on to other players near the "killshot player". so when the walker is close to death, get near each other and finish it off so both of u get credit.