Area 51
Defeating the Theta
To defeat the Theta easily, first make sure that you have a fully charged Mutagen bar and at least three quarters of your health remaining. There are health power-ups lying around where you fight it. Then, enter the room where it is as a mutant and either fire parasites or melee the Leapers that come after you. Use parasites on the first few to restore your health, then attack the rest with melee to keep your mutagen full and also leaves the health power-ups available if needed. Try not to hit the explosive barrels as they will almost certainly kill you instantly. The Theta will appear after about four Leaper waves and leap at you. Fire a few parasites at it, then move in close to melee it. Though the Theta is both fast and strong, in your mutant form it is very easy to hit it and dodge it's attacks because of how the "mutant vision" slows him down. The only attack to worry about is its jump attack that will cause almost fatal damage, even with the damage reduction of mutant form. If you repeat this process (attacking the Leapers with melee or the parasites when they appear) you will defeat the Theta fairly quickly and still have plenty of health and mutagen remaining for the next level.
A good way way to defeat the Theta is to first mutate. Next, fire parasites; or if you want to take a chance, fire contagion to lay some heavy damage on him. Note: Use contagion for heavy damage, but parasites also work well, especially because they heal you. When your mutagen runs low, run up and melee him and/or the enemies he releases on you (the "bubbles" or leapers). Keep doing this and he will eventually go down. This will take some time to accomplish and may be difficult at first.
