ok, let me tell you my plans as far as I have thought about them.
First: I know that I need to separate the 3 big bisses and fight one at a time. Should not be a greater problem.
Second: I know that they spawn a lot of minions. If I want to concentrate on the boss, there must be a way to get rid of the minions or to distract them. Therefore I will either use Windforce with a DOL rune, which causes monsters to flee (hopefully out of range), but my first weapon of choice is silence in a nice bow, that has DOL also and additionally "hit blinds target". Windforce has knockback so I probably have to try which is better.
As I want to concentrate on AI-Curses I cannot use WWS. AD would overwrite the AI curse.
But the main reason for Silence is 75 to all Resis

which will be useful at least against Mephisto.
Third: My preferred armor at the moment is duress which has CB and OW, both needed badly, helmet should either be Gulli or my own kiras socketed with a nice jewel. Maybe the latter to get rid of Minions and for Meph and the first to fight dia and baal.
Rest contains of raven, TG, Dwarfen, boots with CB (what's the name?), crafted gloves with 10% CB or LOH and Highlords for speed mainly.
As I will need torch and anni in my inventory for full resistances I will not use strafe very often, that probably would kick me out of the game. I will only use that to cause monster flee if they are to near.
I also will need to make use of some buildings, causing monsters to spawn outside or the boss inside or something like that, because I only have decoy and valk as good blockers. I don't think it is possible to keep the merc alive all the time. I will see.
As far as I know, CB is halfed on ranged weapons, so I will need double time as with a melee char. But, unlike a ninja or even a Summoner Merc the GA always hits, so I only have to deal with monster block in that situation.
My plan is, to separate one of the big bosses, focus the boss on the valk, have the Merc fight the minions and decoy as strategic minion. Then I should be able to alternately have minions flee and fight the big boss with GA.
I probably need to lay some arrows before I even start the fight and I really don't know how to kill the last 10% of the bosses life. Maybe I will do a weapon switch near the end to get Windforce as "last hits wonder".
So far for the theory.