
Second playthrough clear. Accidentally got Io killed and so ended up doing Yamato's route instead. Got the achievements for no free battles and no-one defeated in battle (I had to restart the final map once because that motherfucker Polaris went and fired a supernova off at Fumi before she could move the first time).
Tried fighting Belzebub but with no free battles and not enough decent demons (I was saving for my third playthrough) I got owned upside-down.
Will fight the secret bosses on my third playthrough.
Edit (August 2012): Oh wow I just noticed that I only tweeted my third playthrough's progress and never posted it here.
You can see my tweets (in Japanese)
here.
I ended up beating Polaris again in my second playthrough to get enough points (690) for my third. Perks I unlocked were skills, macca, auction, one demon, EXP/level limit, half price, new enemy on Tuesday.
I used up most of my demons and macca creating Satan...
...and ended up taking on Lilith with just him and a bunch of low level crap demons. Used the low level demons' skills to teleport over to her and take her on with Satan. Easy fight.
Belial was an easier fight (he uses fire like in Debisaba 1, note my immunities).
Ended up defeating all the secret bosses on my third playthrough. They really weren't very tough at all, especially compared to Lucifer and his stupid megidoradain in Debisaba 1.
Never did get round to a fourth playthrough. I'd like to start on one right now but I seem to have misplaced the DS card. Gah.