Here is the full list of all 8 Remnith achievements.
Kill an enemy unit while airborne
Launch yourself 150 meters into the air by dashing off a ramp or getting knocked back
Find all video data fragments
Clear the final arena on the highest difficulty
Kill an enemy unit from 170 meters away
Kill an enemy unit at point blank
Find a double cannon in level one or a triple cannon in level two
Clear a level two or three arena without getting hit once on the highest difficulty