Making good bosses is hard - the idea behind them has got to be varied and fun while still presenting a challenge, and there's a lot of programming for a small section of the game. Ideally, I think they should seem overwhelming at first but then become easy as you watch their movements, learn the openings and start to fight back effectively.
For people who allow their progress to be updated to the Crystal Towers 2 site, I keep track of how many times bosses have won agains the player and how many players have defeated them. "Score" is [times won]/[times lost] - I think that 2 is a pretty reasonable score to expect, as it implies that players lost to the boss twice before beating them on the third attempt, a decent balance between presenting a difficult enemy and keeping the game going.
It looks like Beam Stack is still the hardest boss, although not by as ludicrous a margin as before - in the first version of the game its score was up at 12 or so. All it took was a little change - making its laser turrets to flash before firing made it much fairer and I think it's one of my favourites. The next hardest one is Xenon Squad, which is fought in a different way from everybody else - its high score might be because players are spending lives working out what they're meant to be doing, though I tried to hint better at this in the new edition.
At the other end of the scale, The Cleaner is exactly where it's supposed to be - it's an introductory boss and even though I actually ramped up its difficulty this time around, it's sitting comfortably near 0.5 (you'll probably win against it first time, but if you don't, you'll get it next time). The absolute lowest is Lava Tank, which is easily my least favourite of the bunch - I think it was the first non-platform-style boss I made for the game and it just wasn't challenging enough despite my attempts to make it more acceptable for the new edition.
There are a couple of surprises as well - Megacarrot, despite being the last boss of the game, is sitting slightly below The Cleaner in terms of difficulty, although it could be because only the really dedicated and powered-up players get to him. I made an effort to drastically rework Sci-Fly compared to the first edition because it was too easy as well - it's only got to a measly 0.83, but I think this is actually much higher than before.
And Cubombscus is at a very respectable 1.81 now despite being one of the easiest before. In the first edition it had a bug that meant that it always remained on its easiest setting no matter how far you got its health bar down, a mistake corrected the second time around!