Upgrading
In order to upgrade your aliens, you must collect DNA points throughout the game. You can then distribute those collected DNA points amongst your aliens to upgrade their skills and abilities. Each alien maxes out at a specific DNA point total, transforming into an Ultimate Alien, or the strongest possible version of that alien.
Requirements
You can upgrade 10 aliens to Ultimate status. Echo Echo maxes out at 110,120 DNA points, making it the quickest upgrade. Ampfibian, Spidermonkey and Terraspin max out at 122,720 points. Upgrading Big Chill, NRG, Swampfire and Waterhazard requires 125,940 points each. Armodrillo goes Ultimate at 127,120 points. Finally, Humungosaur requires 129,160 points. While defeating the game or entering the cheat code "classic" unlocks the alien Fourarms, you can't upgrade him to Ultimate status.
50-Hit Combo
Level Two, the Eiffel Tower, presents an opportunity to perform a 50-hit combo and quickly reach one million DNA points. After reaching the level's save point, use Swampfire and go to the stairway on your right. A large group of enemies will approach. Alternate between regular attacks and special attacks to string together at least a 50-hit combo and upwards of 200,000 DNA points. When you're done, walk into a toxic spill with anyone but NRG and kill yourself. You'll restart at the save point and can replay the same battle over and over, but you keep the earned DNA points each time, making it easy to reach one million points and upgrade all aliens to Ultimate status.
100-Hit Combo
On Level Six, the Amazon, you can pull off a 100-hit combo in the boss fight. Use Big Chill and run to either side of the screen and stick close to the wall. A large brute will attack you with a massive punch while a skinny villain runs along a balcony tossing explosives at you. Focus on the brutish guy, because he's invincible, which allows you to string together a 100-hit combo. Launch five or six regular attacks and then block the brute's punch and the skinny guy's explosive. Now fire five or six more regular attacks and block again. Keep up the same pattern until you reach a 100-hit combo. If you die, you start at the beginning of the boss fight with all earned DNA points. Play the fight as many times as you want to get the necessary DNA points for Ultimate upgrades.