Here is the full list of all 24 ALLTYNEX Second achievements.
Visitor Defeated
Mission accomplished
Ophiuchus Husk Defeated
Alacran Defeated
Ajattara Defeated
Score Over 6,000,000
Adoni Defeated
Zoldyzant Defeated
Score Over 3,000,000
Encounter the Omega Javelin
Shoot down 1,000 enemies
Obtain 25,600 point combo bonuses 10 times in a row
Score Over 1,000,000
Aseliel Defeated
Slash an enemy with a 16x multiplier
Shoot down 100 enemies
First campaign
Type-B Craft rolled out
Return Shot Mode unlocked
1 credit cleared
1 credit cleared with Type-B Craft
Shoot down 10,000 enemies
Score Over 10,000,000
1 credit cleared at Normal or higher difficulty with return shot mode on