You have to fulfill the following challenges to get the achievement:
Promoted to Sergeant
- Total Play time of 2hours+
- Finding and destroying the weak spot of the Matouschka Boss
- Finish any stage with a hit ratio(kill ratio would be a better description) over 85%
- Destroy 50 ships
- Destroy a boss with 90+ seconds on the timer
The weak spot can be found on the last section of the boss. Once you are there hit RT with Speed Up equipped and you can target a part of the wheels at the front. Destroy it while in Slomo and you are done. Easiest way is via Boss Training with everything set at maximum.
Ships are the larget enemies, which take more than 2 hits to be killed. Just replay any stage as you like. For example there are two bigger enemies in stage 1 before you encounter Kolobok the 1st time.