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Is there a way to fight this boss without having to deal with the white robot that appears right next to it? It's difficult enough trying to beat the boss with its1-hit kills, but how is the player supposed to focus on the boss with the other robot?

Also, does the player need to be a certain level to beat this boss? How much grinding is needed in the game?
What "Spinning Robot Boss" are you talking about?

Pax? That robot boss does not spin and is not joined by adds.

Firebug? Spins and flys, but no adds.

Big Sister 1/3? This boss is so big that you are litterely fighting on it. Does not spin, is not joined by adds.

Black Ceribous? Tought critter, but not a robot. It is joined by a fully functioning Pax, but as an interlude to the security boss rather than at the same time.

The final boss? Long time since my last playthrough and have not gotten to this one on this playthrough, but no adds that I recall.

Do you remeber the area this fight takes place in? Does the boss actually have a health bar at the top of the screen? No health bar means that it is not a boss.
Maybe he speaks about a mid-boss, like the Smelter things. It spins somtimes, and there are some who have enemies nearby.
Yeah, IronArcturus is presumably referring to the Smelter boss. IIRC, the best way to fight them is to get up close to them because that puts you underneath the arc of their crane. The problem is getting there; IIRC sliding might make it easier. It has been quite a while since I've played the game, though.

The white robot is presumably a reference to one of the DLC enemies, IIRC it's called the ANGEL gear, it looks like a modern NASA-style space suit. As far as I'm aware, he'll always be there as long as the DLC is installed.

Oh, and grinding won't help you. Armour values don't seem to do anything in this game; damage and stamina are much more important as it lets you dodge and put the hurt on enemies while they're recovering, while health booster implants serve as a buffer to help you recover from mistakes.