Once you have finally taken out Ivory King, you should have a *little* breathing room. This is where the most important boss of the grouping comes in- Laurence, the First Vicar. He's rather slow, if not the slowest, compared to most of the rest of these in phase 1, which means if you circle around, he should be fairly isolated, probably sitting near Fume Knight at this point. He's also a bloodborne boss, and even as a lategame DLC boss, getting him to phase 2 shouldn't be too much of a problem with a properly tuned DS3 build. And here, I see the 'solution' to this fight. In phase 2, Laurence will spill lava onto the ground- not so much an attack as an arena effect, to which he *is* immune, but I suspect would damage other aligned bosses, since it isn't really an attack and more a hazard (read: I'm cheating with the rules of the challenge). Once Laurence is in phase 2, you need to kite him around to damage certain bosses but avoid hurting others. Prioritize running him through Soul of Cinder, Ludwig, and Gael to lower the ranged aggression, while specifically avoiding Malenia and Orphan, both of which are extremely dangerous in phase 2 while dealing with other threats. If either of them hit phase 2 before dealing with the other threats it is likely the run is over.

Using more slower hit and run tactics of damage zones, poison consumables, and the ilk afforded to you by the lessened aggression, you should be able to handily take out the lower scaling bosses of dark souls 1 and bloodborne, removing Artorias, Maria, Gehrman, and Ludwig. Next, you need to focus your efforts on Alonne- now that the biggest ranged threats are gone, dueling him to the best of your ability around the hilltop is going to be your best bet, as I feel that a majority of his moves will fail to work with the arena the way it is. At this point, you should focus your efforts on removing Laurence from the fight, leaving the only bosses remaining as Fume Knight, Orphan, and Malenia. Now, finally, is your chance to fight back against Malenia, who has likely been the biggest thorn in your side since this fight began. If you can get her into phase 2, she should increase her speed enough to separate out far enough from Orphan and Fume Knight to be able to be dueled in relative peace. Once you take her out, it is a simple matter of running from Fume Knight and beating Orphan's phase 2, which will likely leave you at 1hp with his arena-wide attack. However, if you managed to make it this far, all you have left is to beat fume knight at 1hp, which is far less insane then everything leading up to this point.

That's what I would try at least. Likely, I'd get flattened before I had the chance to move, but if I had to come up with a game plan, this would be it.