Quick way to complete the “Frenzy!” achievement.
After starting Frenzy mode from the main menu, you can actually quit, go back to the main menu, and start Frenzy mode from a specific chapter (or at least, for me, after completing the first stage).
At this time, just completing the final stage and defeating Dr. Votsit on Frenzy is enough for this achievement to pop (you’ll see it pop up in treasure room cutscene).
Tips for Dr. Votsit on Frenzy!
Since Overheating and Stamina are no longer an issue, continually jumping/hovering in the air with the blue laser gun does the most safe DPS and keeps you out of reach from the spawned enemies from Votsit’s 2nd phase. Rockets for 1st phase if you’re having trouble.