After lots of judging and criteria-quantization we have decided upon winners of the Sipho Boss Design Contest!
Judging was not easy. To reach the decision we have tried to imagine how your creations will work in various game scenarios. Less violent boss fights, based on boss behaviour and its "decision making" were evaluated more highly. Some entries even felt like they have been mind-read out of our design ideas list and that gave a lot of confidence in both ideas and entries. We also looked at additional mechanics we would have to implement for some of entries to work and how well we could re-use them in the rest of the game. More re-usable game mechanics were rated more highly. Let's get to the business.
First place winner is:
Untitled Maze boss by skipydog

You are in a maze with 4 - 6 boss Siphos. Each one aimlessly wander around until you are in their line of sight, where the one that sees you would chase after you. You have to maneuver the maze and not be seen by them, they can only see from their front side. You can safely attack them from the sides and easily kill them that way, but doing so will make all others seek to where the kill was, so you have to be careful.
When you wither it down to around 3 or 2 left, the rest will flee into a whirl pool and you win the fight.
You should not attack form the front side of them.