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[SECOND EDIT: I have unlocked The Arena - turns out, the reason I didn't unlock it was because the cutscene progression seems to be botched due to me skipping Boss 11 - after I beat Boss 11 (I face-tanked the "overwhelming" phase - didn't have any idea how I would dodge it otherwise) then Boss 12 (after a few tries, due to that damn Hypernova having the ability to OHKO, and also because I was going for Silver Medal, and that timer for Silver Medal is nasty), The Arena unlocked]

Thanks to the timer increase, I managed to gain the Silver Medal from Boss 9 after a few tries, with less than 10 seconds on the clock. I also managed to get Silver Medal for Boss 10 after a few tries, with a mere 5 seconds left on the timer :)

[EDIT: I just beat Boss 11, still not a fan of the "overwhelming" phase at the end - no Arena unlocked, though, so I assume that it is locked behind medals?]
I don't have The Arena unlocked, which I presume is either because I don't have Bronze Medal for Boss 11 (never managed to beat it, sadly - I usually get done in by the second form even before the "overwhelming" phase), or I don't have enough medals
(I have 35 medals:
- 11 bronze for beating all the bosses except Boss 11
- 10 silver for beating the first 10 bosses on Timer Battle
- 6 gold for beat bosses 1, 2, 3, 4, 5 and 7 on Danger Battle
- 4 Pix medals for beating Boss 5's minibosses separately
- 4 blue medals for beating Mini-Game Quests 1-4)
[EDIT: I now have 36 medals due to having beaten Boss 11]

I just noticed that Boss 6 (specifically the Danger Battle version) slowly regenerates its HP, which I suppose is a measure to force the player to fight him honorably instead of coward-ing by occasionally firing Bronto Burt shots from the distance - that's a creative solution, I kind of like it(?)

For the "some attacks are harder than others, so the boss is a bit RNG" problem, I've had a bit of thought about it. I think I've noticed what you mentioned about Boss 10 and Boss 11's second form having that "weak attacks -> strong attack" pattern, and funnily enough, I think that HAL also reached a similar solution to that with the Knight bosses in the later games (since those are also somewhat random). Looking at WiKirby's page on Morpho Knight reveals that its attack pattern follows a 'script' of "do a few random unscripted attacks -> big scary scripted attack".

Btw, is it intentional that upon entering Boss 11's second form, the player's HP drops to 5 or 6?

[SECOND EDIT: The timer for Boss 12 is rougher than one might initially think, due to certain attacks which can't or are difficult to punish, such as Hypernova - in order to get Silver Medal, I had to face-tank a few attacks towards the end to keep consistent damage, which paid off since I got Silver with about 30 timer seconds remaining. Onto the Arena! I'm looking forward to this, since I'm an absolute sucker for Boss Rushes]