WARNING: your multiplayer ranked kills DOSE'T save. Just like every grind achievo in game you have to go to campaign, press start and make a MANUAL save in there. Then multiplayer kills as everthing else towards you progress saves. This means you dont need to grind all 500kills in one go.
Easely can be tracked by in game menu- stats -achievments-medal of comendation. Can check it your self, get atleast 1kill in ranked game, finish a game, go to dashboard without saving and what will happend-progres lost, with SAVING progres there.
How to do the kills, dont waste your time searching maps and modes posted above solutions. Chose RANKRD small team game or large team game to work towards "all purpose warrior" achievment, and click find custom mach. Since they are random any map will work or mode as long as you grab as sniper or bow and 1shot kill enemy from distance in there spawn area. Enemy guy just comes out of hes spawn in shooters sight and dies. Reapeat.
If you get capture the flag mode even beter cuz can go for 20min for unlimited amount of enemy kills.
Give a like if it helps and makes thigs clearer! :)