Like a lot of other folks, I'm really impressed by this entry because of the quality of work done by a relatively-unknown person. Besides that, taken on its own merits, it's a cool sprite. Plus, Undines rock, and so does the Seiken Densetsu series.
If anyone can't pin down why exactly this boss is so nice, I'll spell it out.
The following attacks are boring:
- Spam projectiles.
- Spawn generic enemies.
- Charge blindly at you.
You'll notice that Undine only throws one projectile, and has an attack animation. That alone elevates it above 90% of the custom bosses out there. Next, the wave forces you to move around, which is something every boss should do. Finally the icicle attack isn't a projectile or charge, and has an attack animation.
Some additional behavior would be nice, but with only 3 hp the boss won't get tiring.
TBH, there is no real inspiration.
Some time ago I had an idea for a complete mod with a story which included the different spirits from SoM. But I figured I probably would never be able to finish it as it was quite ambitious and obviously would have been my first project.
However I did not want to abandon the ideas I had completely and tried to realize at least this. So yeah, in that Hack Undine would have been a boss. Maybe I'll actually start that hack if I find some spare time, first thing would be to write down everything that has to be done in some kind of design document.
Regarding the boss directly, my thinking process was probably like this: I tried to answer the follwing questions and goals:
1. I need more than one attack to make it more interesting.
2. I need some way to hurt her.
3. There should be some gimmick to it.
4. I should be fun and challenging but not unfair.
The first part was answered quite fast by thinking about the SoM series. I decided to implement one indirect and one direct attack and came up with the projectile (internally called frostwave) and the spikes (internally called icelance).
The second one wasn't so easy. Obviously there were two more or less common ways. Jump on her, and throw something at her. As I thought it was unapproriate to make her be on the bottom of the screen and therefore unreachable to jumps, I decided to get something to throw at her and I already like the idea to use something from her against herself. This is when I thought about the tidal wave to stop the iceblock from being destroyed.
Luckily the same idea fulfilled goal #3 so I was very satisfied with my current design. =)
As this was my first project I could not really comment on the difficulty beforehand and just implemented it. I made lots of defines to make it adjustable to finetune the difficulty later on, e.g. the speed of the different parts, the time, the health points, etc.
Comments here made me realize I probably overestimated the difficulty and added the speed increase to the tidal wave. (Sidenote: In the beginning the tidal wave disallowed you to jump when you touched it, but I decided this was unfair. ;))
That is basically how I came up with this boss.
Another honorable mention should obviously Brutal Mario, which impressed me a lot with what is possible and made me interested in SMW hacking in the beginning. (Probably because I really like the SoM series and there are so many references in BM. =))