Sorry, but i really doubt it since most of the stuff you wrote is wrong (vorpal/black blade of disaster+ make em to near death when they heal themselves) or useless (unless you start being more specific than of "izi pizi just try to focus and good coordination"). Or maybe you managed it but with some buff mod. The only one I am using is bottomless bag of holding FYI.
Unless something was changed very recently, I doubt that BBoD doesn't work at all on the Five. On top of its vorpal chance, BBoD has a 2k12 damage spread and is a +5 weapon, so you're really meant to use it to obliterate something fast with IH / Mislead for constant backstabs; I explicitly remember using exactly this method of obliterating Illasera and Sendai.
Had we had power mods in play (which was never my assumption), I would have told you to get Water's Talon +4 from IU or Pontifex's boots from Tactics for maximum cheese, but none of that is the case, so I didn't mention them.
Do you have a Simulacrum source? Do you use it as a bonus repository for all your crucial (green) scrolls and a secondary tank?
Do you sic the Planetar on Balthazar for at least a little while? Illasera or Sendai might not look like they're trouble, but Illasera has arrows of dispelling that lower your resistances. They're easy to dispatch anyhow (Sendai is very fragile to Remove Magic).
Do you use Protection from Magical Weapons? How about Hardiness when you do actually come to blows with Balthazar? With a proper setup you can achieve massive physical resistance with Hardiness. Do you have Defender of Easthaven for your off-hand?
Do you use the special Ascension ability, Focus, to be able to abuse Time Stop in any capacity at all? Your Simulacrum preserves your Focus ability if you cast it after you use Focus, so you can have 2 F/M/Ts wreaking havoc on enemy lines, only having to pay attention to Balthazar.
Do you pay attention to the action feed at the bottom? Balthazar's immunity to Time Stop is gated by his Lunar Stance, and only him and Mel should be ever immune to TS.
Do you go into the fight with good Contingencies and Spell Sequencers to be able to soften the targets up?
Try doing this: prepare a Planetar + some random fodder summons (with really good timing you will stop time in such a way that only Balthazar will be moving, and he will waste time punching your immobile summons to death), Focus + Hardiness on your main hero, then Simulacrum, green scroll of choice on enemy, Time Stop, and go nuts. Kill order is up to you, if you don't see a point of killing Illasera first (though, again, her arrows of dispelling are a bitch, so get rid of her at *some* point), go for Balthazar.