The Frenzy Meter is a score-booster that increases with every fish / starfish you eat. Each time you eat a fish / starfish one letter gets added on until FRENZY! is filled up, at which point you've got a Feeding Frenzy. If you go too long without eating a fish / starfish, the meter decreases. You must fill up the Frenzy Meter five times without it decreasing to get this achievement. That means eating 35 fish / starfish.
It's much easier than it seems though, as I got it in the first level after about 30 seconds. Just use boost a lot and grab all the fish and starfish that you can eat.
Later in the game you can grab FF (Feeding Frenzy) bubbles which give you an extra Feeding Frenzy bonus. So in theory, you could fill up 4 sets of FRENZY! and grab the FF bubble to finish it off.
These are the levels of the Frenzy Meter:
Once you hear the words "Mega Frenzy", the achievement should unlock.