Tip for hard survival:
Kill all enemies except slimes, they will soon bundle up behind you and fill the mob limit.
Although the limit increases every few kills, spawning 2 monsters instead of one, you won't have much trouble taking care of 2 at once while running in circles around the slimes.
If nothing new seems to have spawned, it's all slimes now, so kill a small one and look around.
That was the only way for me to manage over 400 kills in hard survival mode, although it took some time.