I have created this first boss fight in last two days.

Boss - Mechanics

There is two phases at the fight. First one is the clasical slowly walking, hitting hard and slow. But when you make his health go below %60 and %40 he will cast a summoning spell. He will jump back a little bit and become invincible at this phase. He will drop cactus from above player for 10 seconds.

Level and Boss - Design

I know it's a fast change from dungeon to wild west train fight. But this was the idea when i started developing this game. I want a big changes every time you go for a boss fight. So i started creating a level for you to fight play. I made the arena 3 vagons long which i think is way to big for now. After finishing the platforms i added the parallax background and wheel animations. Then only thing i needed to make was the boss himself. I wanted the boss to be a cowboy-cactus. So i started drawing it. I made an idle, walking, hitting and summoning animation for now.

I am planing to add a gun shot skill to him as well. I am thinking if you get hit by it you will bi stunned for a while.  I am not sure of adding a leash skill for him that he can pull you to himself.

Final Notes

I will be adding 3 or 4 more vault levels and a boss after every vault. I haven't planed the themes for those levels completly but i have a few ideads.