Best weapon to fight him with?

I used 2 summon mods for this fight, and he definitely killed one of my little Hollows at some point. Still got the Void Sliver, so I think Claviger can't absorb your summons on current patch.

He do not absorb the dog from the weapon mod

For ME this boss was actually fun, and fair feeling.
You can TELL when he's gonna summon his ads. They don't just pour in. It can still be kinda overwhelming, because they're powerful Ads, but seeing the boss actually take time to summon them instead of being bum rushed from behind feels substantially less cheap.

Turrets made it much easier for me. Distracted the boss so I could quickly kill adds.

jeeez this boss is fckn stupid on normal... been trying to get that melee weapon for ober an hour and more than 10 attempts now, but he either kills his minions with his laser himself or the kill each other shooting at me -. - either way they're dead before he can absorb them... i even tried not shooting them myself to make sure HE is the one killing them, but nope, guess I'm stuck with the particle accelerator^^

why did you put "semi optional"? it's not optional in any way. You must defeat the world boss to get the key. there is no other way.

From my experience if he absorbs even the corpses of his minions it stops him from dropping the Void Sliver. I've experienced this multiple times, it's quite annoying.

The page description makes it sound like both bosses will spawn in your world this is not the case or I got unlucky with my map he was my only option there was no harrow during my playthrough, also a tip if you are after the void sliver bring the goodest boy perk he decimates the adds

The bridge to the claviger fight looks like a p3nis and b@lls