To Build Automated Kill Area
Building a spawner is fine. Getting it automated is even better.
I will tell you how to build an automated kill area / lootcollection that works for everything besides Warmworms. Not sure about Rockzillas though they seem quite stationary and i never build that spawn as of now.
First, start off by making a room, which cant be jumped (4 blocks high without roof). Only use a roof for creatures that need darkness.
Then take out 2 blocks in hight at base lvl and put a teleporter there. This is where your creatures spawn.
Build another room at least 3 blocks high and fill it with corrupted water - or in case of corrupted creatures - mineral water. Make an entrance 1 block above base lvl with a door and one block to stand on inside, so you can overlook the room. Place the other teleporter in a corner and connect them. This is where they get killed.
Why It Works
The idea is that creatures will try to esape the room they spawn in if possible. By limiting the space to walk to the teleporter, they will inevitably go there, end up in the water deadly to them, and die.
This was tested with Mirus, Hot Feet and Reinbous so far, but all moving creatures share this behaviour.