Here is the full list of all 25 Baldr Sky achievements.
Complete Chapter 1
Complete Rain Route Ending 'Rain'
Complete Rain Route Ending 'Ghost'
Complete Nanoha Route Ending 'Nanoha'
Complete Nanoha Route Ending 'Dream Land'
Complete Nanoha Route Ending 'Embrace'
Complete Chinatsu Route Ending 'Chinatsu'
Complete Chinatsu Route Ending 'Entwined'
Complete Chinatsu Route Ending 'Departure'
Complete Aki Route Ending 'Aki'
Complete Aki Route Ending 'Ark'
Complete Aki Route Ending 'Nirvana'
Complete all reminiscence events.
Complete Makoto Route Ending 'Makoto'
Complete Makoto Route Ending 'Object Loss'
Complete Final Route Ending 'Sora'
Complete stage 25 on (Dive 1) survival mode.
Complete stage 50 on (Dive 1) survival mode.
Complete stage 75 on (Dive 1) survival mode.
Complete stage 100 on (Dive 1) survival mode.
Complete stage 25 on (Dive 2) survival mode.
Complete stage 50 on (Dive 2) survival mode.
Complete stage 75 on (Dive 2) survival mode.
Complete stage 100 on (Dive 2) survival mode.
Complete the true ending.