When I went for those I had to do only that task, not multiple ones (as for some reason they seem to override themselves)
When it comes to doing them 'easily' I actually dont fully remember my spot, just that it was near the vending machine with my back to the wall, leaving only the one I was going for alive and using all the 'healing' options from the vending machine between rounds.
Also the full auto shotgun is really great for dealing with them quickly, but the secret weapon on the stage is actually better for making sure you dont kill the one your aiming to keep alive.