Brothers, United features the battle between Sage Mode Kabuto and the Uchiha brothers Sasuke and Itachi. This battle is a bit tougher than the ones in the main game, so I'd advise you have the best bento that bolsters both your attack and defense that you can purchase from the Village Hidden in the Clouds. You can warp here by talking to the ninja that is in the central area of the Hidden Leaf Village, in the same area where Shikaku rates your Ninja World Timeline.
Speaking of the timeline, you'll want to complete the battle "Survival Drill" if you're booting this up after already completing the base game. I'm not sure if this was the case for everybody, but "Brothers, United" didn't unlock for me right off the bat so I went to complete Ninja World Timeline battles I hadn't done. The first one was survival drill, I did this and it unlocked the DLC chapter immediately.
I'd also suggest using a Hero kit and stocking up on the best health recover items available. Kabuto is fairly easy for the first portion of the fight however the second half he takes quite the beating so it pays to be able to keep your health up. Normally I'd suggest the pills that up your attack, however health is more key when going for the S ranks.
For the first portion of this fight you'll be playing as Sasuke against a standard Kabuto. This portion shouldn't be anything special, really. If you've used a decent bento this should be easy enough. After you've brought his health down he'll recover and you'll have to play through the quick time event. Be on your toes, the better you score the more health you gain back.
The second portion of this fight is quite a bit tougher. You'll be facing an awakened Kabuto. He also has a few moves that are special to this occasion featuring members of the sound five. Each of these attacks is fairly predictable given the fact he jumps up on to a higher surface before executing them.
First, he'll use Kimimaro to charge at you three times. Simply ninja move to the left or right and it should be fair enough to dodge. Jirobo will move chunks of ground towards you. You'll have to analyze these and look for the opening and ninja move away from where they will slide. Sakon is constantly a part of him and Tayuya sends a stream of ghosts after you one at a time. Lastly you have Kidomaru who sets up web traps along the ground that you must avoid. These all come in waves until he gets low on health, then it's two or three at a time. When he's on his final bar of health, make sure to use whatever health recovery you can for the best result possible.
Generic tips for S ranks...
Health is a big part of the ranking system. It's pretty much the key statistic.
In stages with mob battles, you'll want to defeat a few extra enemies that you wouldn't normally have to. This is because you get credit for particular milestones like 30 and 50 enemies defeated and for landing so many back attacks. The back attacks are when you defeat an enemy with a melee stream and then tap "X" to end up behind another enemy. I'd suggested getting a few extra kills, just to bolster this rank. This is particularly true in "The Seven Swordsman" because if you don't go out of your way to defeat extra zetsu while on the Her path, you won't get the S rank. Thankfully for this mission you only have to worry about the boss battle.
Stars Collected in the quick time events are also a big part of it. You'll want to do your best to get as many as you can. They also help restore your health, making the S ranks easier.