The conditions you must absolutely meet are as follows:
- Sit with Tatsuki at the welcoming party on August 1st.
- Set up a meeting with him on August 2nd.
- Choose to visit Tatsuki on August 6th.
- Select the option "Somebody help me!" on August 7th.
As long as you meet those requirements and do as much to gain Tatsuki's affections, he won't die.