CONCEPT FOR A "The Devil's Amalgam" boss rush:
Phase 1: 9
It starts with the regular 9 cutscene (uncovering 9, it growing spikes, ramming captain smiley etc), and it lasts up until 25% of the board is cleared, at which it will disappear (you do not need to click on 9), leaving room for..
Phase 2: 10
right after 9 disappears, 10 (boss form) wil immediately appear and start its cutscene from the "ramming smiley" part, and the cutscene will instantly transition into the phase itself. It lasts until another 25% of the board is cleared (aka completing 50%), at which it will meet the same fate as 9.
Phase 3: 11
once 10 disappears, all flags are immediately removed. 11 will then appear and instantly start placing flags just like how it does in its bossfight (like, 10 or so flags?) before doing its normal in-battle behaviour. The mine handicap is +10 (only when 11 is active tho!). The phase lasts until 40% of the board is cleared (total: 90%), at which 11 will not despawn
Phase 4: calamity
9 and 10 shall appear (their timers/values are reset) in the positions they were before they got disintegrated, and it's just the finale. Complete the remaining 10% and then click 9 to winga.