Story related and cannot be missed. Before defeating the boss of this level, I'd suggest reading the "100 Hit Combo" achievement description. This is the best location to get 100 hits but you will need to do it before killing the boss. If you don't do it on your first run, you will have to at least replay this entire level.
Once you defeat the bosses of level 6 the achievement unlocks during the cut scene that follows. To defeat these guys, you need to first use Ampfibian to shock the floor pad when the skinny guy is on the ledge above it. The fat guy will then charge the wall and make the skinny guy fall. Run over and punch him at few times. Now switch to Echo Echo and when the skinny guy is over the 2 switches call in your clone and flip the switch. Once again the fat guy will knock him down so start the beating again. Now on the third and final run, switch to Big Chill and you guessed it, freeze the flame when the skinny guy is over it. Once the fat guy knocks him down, run over and finish the job.