First I would like to say in order to get the counter option to happen here you can easily just mash the block button (RB I believe) in order to get the first prompt to pop up (It's always A). InLast has a great solution for this with the push start upon seeing the prompt and then mashing it after pressing B to exit the menu. There is an infinite spawn when you are siding with the Ouka clan mainly for the ending where the boss leaves and you must try to protect the minori castle there and then escape. After the first wave where they come through the gate move forward until Kirie attacks you. Now, while ignoring him for the moment start a chain and run around this small area by the gate. The enemies should run in from all angles however you want to be looking out for enemies after you chain kill the group that just spawned because there isn't a whole lot of time you maintain the counter state and once it goes your counter starts over. This way you can get an easy ending and what I found to be the easiest way to chain 100 kills. Best of luck!