I've killed them without the Divide & Conquer tactic once or twice on Insane though. I did of course take out the demon and golem before engaging, but I don't really consider them to be part of the team, since Ilyich and his crew doesn't react to anything they do.
Did this with Imp Ilyich v10 with a PC Ranger/Cleric, Minsc, Imoen and Yoshimo, also without resting
I wrote a more thorough report about it on Ironworks, but here's the Ilyich encounter part:
No big preparations could really be done, Yoshi planted a trap outside the the visual range of Ilyich's party. Minsc used the other Oil of speed and we then both removed our armors and hid in shadows. Imoen got a couple of Kobolds out of her Monster summoning spell to serve as cannon fodder.
Minsc and I moved in behind the enemy party, Zhivago began casting True Sight, but Yoshi tossed a trap from a safe distance right smack into their party, this one slowed Karamazov and Zhivago. Minsc and I still remained silent while Rasputin and Ilyich charged towards the lone Kobold I had placed in front of the other trap, which also absorbed Ilyich's annoying Classic Blunder ability. The instant they moved away, we revealed ourselves and began hacking away like mad at Karamazov. She was still quite deadly, even when slowed and within melee range. Ilyich and Rasputin both got hurt by the other special trap, and now Ilyich was also slowed. Imoen used her first Paralyze wand on Rasputin - and then he was history.
Minsc and I had nearly finished off Karamazov, though she had almost finished me off as well. We had been hacking away constantly for more than two rounds, only stopping once to disrupt Zhivago's Deva casting. Minsc had even used his Berserk ability just to get that extra edge we would certainly need. She finally died, and the instant she did we began working on Zhivago. I now lost control of Minsc due to the Berserker rage, and he had been reduced to 15% Health after attracting Karamazov's attention for a few seconds before she was killed.
Ilyich had now killed the Ogrillon that I also received with the Monster summon casting, which was the last creature I had to stall him with, and now he charged towards Minsc, albeit very slowly Never the less, Minsc targetted the right foe, and as long as Ilyich, who now had regained his normal speed, didn't see what a sitting duck Minsc was, I'd be alright. Imoen tried to Paralyze Ilyich, but he made his save. I had been reluctant to get Imoen or Yoshi near anything hostile due to their very fragile nature, but now I deemed it safe for them to begin hacking away at the disabled Rasputin, who wouldn't remain that way for a very long time unless I finished him off soon.
Zhivago was almost dead, a hasted dual-wielder and a furious and hasted Minsc didn't take that long to chop up anything, even at this point of the game. He died just as Ilyich took his first swing at me. He was barely injured, but that changed soon after Imoen cast MM and Minsc and I had aimed a few swings in his direction. I had to move away though, since he was hurting me at a far more devastating rate than we were hurting him. Which caused him to target the still uncontrollable and badly injured Minsc. Well that's it for him I thought, but no, Ilyich missed him on the first swing, and before he had the chance to swing again, an extremely lucky Critical strike from Minsc, a Lightning Bolt from Imoen, a failed backstab from Yoshi, and two quick swings from my own two war hammers, had knocked him out permanently. Rasputin was still alive, but in his disabled state we quickly killed the last remaining member of Ilyich's crew.
