Here is the full list of all 8 I Got Isekai'd Into a Shmup Demo achievements.
Kill your first enemy.
Make a score of at least 5 000 000 POINTS
Make a score of at least 10 000 000 POINTS
Complete a stage achieving an S rank.
Kill the 1st boss
Kill the 2nd boss
Kill the 1st boss (Manic Mode)
Kill the 2nd boss (Manic Mode)