Mainly because I feel it unbalances the fights, anyone can just walk up and wail on a boss while the other draws aggro, taking away any challenge it would of had, the AI just isn't up for facing multiple people.
If it had better AI and the bosses stats scaled accordingly to the number of players then I'd feel okay about it.
I personally don't like the slim, I got one back when it came out because my phat's disc tray was acting up, the disc drive in the slim is too noisy, you can clearly hear the laser seeking similar to a Dreamcast, and it gets really hot from the lack of a fan, I took it back because I was afraid it'd overheat, used the money to have the phat repaired and have used it ever since (it's hooked up right now).
I'm really looking forward to getting to fight this guy. I've felt that the Souls games, for how good the bosses usually are, have had fuckin' terrible final bosses. All three of the previous games' final bosses I have beat on the first try.
I think you may have been overleveled, but I definitely feel they've gotten less deadly, King Allant was an actual challenge and freakin' badass, Gwyn hit like a truck but was really easy to read, Nashandra was a push over, like unbelievably easy, the twin boss fight just before her would have made a better final battle.