How to can I earn 'Completionist'

[CirclezerO]

Completionist
Finish all of the secondary quests in Mars: War Logs

I played 5 times, but I can't earn Completionist achievement.

please, give me advice.

[enkaku-silence-]

I didn't do it yet, but my guess is that you have to accept as few sidequests as possible.
Save before every conversation to avoid getting sidequests which are hard to solve.
I'm pretty sure, that sidequests which weren't taken, don't have any effect on the achievement.
It's just about solving every quest you have taken, because some sidequests are contradictory which would make this achievement impossible.
Hope this helps.