The best level to do this on is Curtains Down (Opera level). Best thing to do is bring in your silverballers with extra ammo and magnum ammo a SMG again with ammo upgrades and magum ammo and finally your M4 upgraded. Having the painkillers, 35% Kevlar vest and playing on normal difficulty because of the saving system will help a lot. I would start keeping a mental record of the kills.
From the start go to the left of the security guards office and down the stairs there should be a worker who you can kill. Run to the washroom and grab your m4 and start killing the security guards and secret service guys. When you run out of ammo and have no guys shooting at you grab the MP5 from the guards and ammo then make your way to the start again killing everyone you see. Including the American politician and opera singer.
I was at around 30-35 kills here and made a save and exited the level and seen how many kills I had. You can restart the level and reload your game collect the other kills and exit and hopefully get your achievement