The best way to defeat a 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.