After completing the first 5 levels of the game you unlock level 6 which is the "fire boss". You will need 15 DNA to begin the level. The tactics to defeating the boss are fairly simple and repetitive. First, he will saw into the ground, so avoid the saws and the marks they leave behind. Then he will have lasers that move across the screen. Just boost jump over them, and when he reveals his arm with the red mark, boost into them. Finally, he will spit out red, exploding bugs of doom. Avoid them, stack up your guys, and attack whichever eye of his is open. Once that is done, meteors will rain down so collect the points and repeat 3 more times.
Note: He will get more aggressive each time. More saw marks, faster and twice as many lasers.