Pretty self explanatory, just keep swapping weapons from the enemies that you kill. It doesn't really matter which weapon you start with, as all will be found later on as well. I'll list the weapons below that I remember encountering, there might be more.
There's the 4 pistols:
- 50 GS
Besides, these weapons can also be found from the dead enemies:
- FR 5.56
- Model 680
Finally, there are the RPG-7 and the LMG in the back of Nikolai's van. That should be 12 weapons, so getting a kill with 8 should not be too difficult.
Here is a videoguide made by Maka for those interested: