Go to Mephisto’s puzzle area and you’ll have these little creeper things. They’ll spawn infinitely and won’t stop coming until you’ve done the objective. Do not do it and just throw them off. Once you’ve throw over 500, you can then start just killing them to go towards your 4000 kills.
And the second best place is load up Sepulcher and walk to the back over the bridge and there is an an awful lot of enemies you can just throw off, enemies are near infinite on this level as they will respawning throughout the level for a certain amount of time.