For some reason when I try that the combo counter gets reset multiple times in the middle of the attack. Don't know if that's a bug or if the creator patched the game to deny that cheese...
Doesn't help that the game is REALLY finicky about the timing for actually activating that attack.
The exact same thing is happening to me, and I was just coming to check and see it anybody had any advice about it. Based on what Belkezar said, and what I can see from the rapidly changing numbers, I think what happens, is that each tentacle and the boss itself have separate counters when you start blasting them. If it gave a 170+ hit combo on the death blow, I suspect that if you keep the combo going the whole time it might add them all together at the last hit, but if anything interrupts you, like say finishing off all the tentacles and causing the boss to enter his own laser mode and be invincible while he does, then all those extra combos from the tentacles go away and your left with nothing but broken dreams. It's driving me berserk.
Edit: Solved it! Sort of anyway. I'm fairly certain I'm right about what's happening with the combo counter, so what you need to do to keep from activating the laser beam mode and losing all your combo progress is to have the final boss health reduction at 50%. This lets you kill it in two blasts on normal mode (which I recommend using for this, it gets the health to just the right point) and assuming you were hitting all the tentacles while you were blasting him you should get the mission completion at the end of the second blast as he dies.
Of additional note to anybody who cares, while grinding the boss trying to get the combo counter I discovered it was also the best way to grind for Game points to spend in the adjusted mode panel, because you get 200 points every time you beat the boss as a you beat the game reward, so if you've saved right before the boss like I did, you can just blaze away at him a few times and very quickly have all the points you could ever need. Incidentally, this lets you easily unlock any defeat scenes you missed along the way by unlocking the ability to buy them with points and using your new riches to get them. This means that while the game doesn't technically unlock the defeat scenes upon completion, if you've beaten the game, you can effectively do it yourself in short order. There will only be a few missing that have different unlock requirements at that point. I really wish I had realized that before I had worked through all the unlocks the hard way before beating the game.
