This is a three phase fight and it is not nice.
First of all, don't approach this fight like you will win first try. You'll just get pissed off. You need to be able to beat Emma and Isshin phase 1 without resurrecting or taking much damage. Practice with Emma before you get serious about trying to beat Isshin. Yellow or red sugar will help, depending on your approach.
1. Emma- She is fast. I had a bit of trouble with her because she seemed faster than most enemies. Took me a while to get the timing down to parry her. She has INSANE reach on her Ashina Cross and her thrust attacks. Run FAR AWAY when she sheaths her sword (Ashina Cross) and dodge to the left after she slashes and then punish. Double back step or double jump to dodge her grab. Other that, just pummel her with attacks until she parries, then parry her attacks. You want to break her posture ASAP.
2. Isshin phase 1 - Play more defensive than usual. He is good at sidestepping your attack and hitting you with a counter attack that is hard to parry. Instead, bait him into attacking you, then parry to build posture. Handle his Ashina Cross the same as Emma's, but his is actually easier to exploit ( if he did this move more often, the fight would be easy). He also has two different thrusts, one of which is hard to counter until you know what to look for (when he goes to do the new thrust he will always attack once, then sidestep to his left and quickly thrust). He does an unblockable grab that I had a hard time with. Best I could figure out, you need to jump backwards like crazy. He also does Ichimonji Double, when he holds his sword up straight in front of him, he will do two heavy chops. This is possible to dodge, but it's difficult. I found it easier to parry, but your parry has to be spot on. If it's early, then his first attack will stagger you and the second chop will hit you. This phase is slightly slower paced, so you can focus on health or posture, although I preferred posture.
***Practice with these two phases until you can do them without taking much damage ****
3. Isshin phase 2- Eeew. This phase made me rage. He gains fire attacks. They are not cool. Go ahead and accept that you will have to lose this fight a few times to come to grips with his new moves.
This phase has to done defensively. Most of his combos lead into a fire attack that is unblockable. It's best to bait him into attacking, then punish. He will crouch and growl for a few seconds, then the whole floor will erupt into flames! Then he will jump straight to you and surround you with a ball of sword slashes followed by a heavy slash. Even if you block this, it takes most of your health away. It seems possible to parry, but you would need insane reflexes. Easiest way to deal with it is to cancel the attack! To cancel, AS SOON AS he starts growling and crouching, hit him with firecrackers and punish. He will also do a sweep attack that emits a sweeping fan of flames. Jump back and to the side, run behind him and punish. He will also do his Ichimonji attack that will shoot a wall of flames straight at you, all the way across the arena. Dodge to the side, run behind and punish. He throws in regular attacks and the Ashina cross as well. Try to pull him to the center of the arena at all times so that you can get behind him and so you are close enough to cancel the mega attack. ***Firecrackers are super helpful in this phase. Try and save all of your spirit emblems for this phase.
*Shadow rush is a skill that I found helpful at first on the Emma and Isshin phase 1 phases, but I recommend beating them without using it much so that you can use firecrackers as much as possible on phase 2.
*When using firecrackers, wait until he is at the end of a move so that you won't waste them.
*Dragon's Blood Droplet can be used in battle to renew your ability to resurrect. This will give you two resurrections in phase 2 if you need it. Remember that they are limited though! Don't use unless you are sure you will win.
*Your first resurrection comes back at a idol. The other two must be refilled by getting deathblows on enemies, or with a Jizo Statue. Try to resist the urge to resurrect more than once unless you are sure you can win the fight.
Good luck!