There are 5 endings in Tatsuya's route - Mystic, Dark, Passionate and 2 bad endings.

Instead of writing choices for every route separately I typed all the choices down and marked which ones I picked for which end. Keep in mind which end you want and pick accordingly. !

Bad end #2: Go for Mystic end, but when it comes to choosing between Tatsu or Katsu, pick Katsu both times. I indicated a place to save for a quick reload.

Hope this helps anyone struggling to get certain ends. Cheers!
As Tsubasa would say: Your... erm... pants are vibrating.

Achievements for Tatsuya's endings are:

Foray into the Arctic
Cleared one ending in Tatsuya's route.


Hypothermia
Cleared one bad ending in Tatsuya's route.


Frostbite
Cleared all bad endings in Tatsuya's route.


Dragon of Winter
Cleared all endings in Tatsuya's route.

Chapter 1
[Tell him to back off.]
And what if I was interested? || Dark, Passionate
You’re the one standing so close. || Mystic, Bad End #1

Chapter 2
[Introduce yourself] || Mystic, Bad End #1
[Ask him what he wants] || Dark, Passionate
[Accept his compliment]

Chapter 3
No way! || Mystic, Bad End #1
I wish. || Passionate
Yep! || Dark

Chapter 4
[Try to get his attention.] || Mystic, Passionate
[Ignore him] || Dark
[Yell at him] || Bad End #1

Chapter 5
[Explain] || Mystic, Dark, Passionate
[Say you don’t know] || Bad End #1
[Refuse until he explains]

Chapter 6
I’m not. || Passionate, Bad End #1
I’ve seen him around school.
Why are you so nosy? || Mystic, Dark

Chapter 8
[Warn him against sneaking in] || Dark, Bad End #1
[Thank him for all he’s done] || Mystic, Passionate

Chapter 9
I understand. || Mystic, Dark
What do you call it then?! || Passionate, Bad End #1
How could you do that to me?

Chapter 10
I don’t know what to say.
Too bad, not interested. || Dark, Passionate
...You have? || Mystic, Bad End #1

Chapter 11
...Maybe you’re right. || Bad End #1
No, please wait – || Mystic, Passionate
I just need some time. || Dark

Chapter 12
I don’t need to. || Mystic, Dark, Passionate
I need some time.

Chapter 13
(make save on Mystic route for Bad End #2)
Tatsuya. || Mystic, Dark, Passionate
Katsunoki. || Bad End 2
Neither.

Chapter 14
You. || Bad end 2
Tatsuya. || Mystic, Dark, Passionate
I can’t choose.