To defeat the Theta easily, first make sure that you have a fully charged Mutagen bar and a minimum three quarters of your health remaining. There are health power-ups lying around where you fight it. After that, enter the room where he is as a mutant and either fire parasites or melee the Leapers that come after you. Using parasites on the first few to restore your health then attacking the rest with melee works well to keep your mutagen full and also leaves the health power-ups available if needed. Avoid hitting the explosive barrels as they will almost certainly kill you instantly. Theta will appear after about four Leaper waves and leap at you. To defeat him easily, fire a few parasites at him then move in close to melee him. Though he is both fast and strong, in your mutant form it is very easy to hit him and dodge his attacks because of how the "mutant vision" slows him down. The only attack should worry about is his jump attack that still deals 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 him fairly quickly and still have plenty of health and mutagen remaining for the next level.
-A recommended 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 patience and time to accomplish and may be difficult at first.
-If done correctly, its easier to not mutate in this battle. Save up plenty of shotgun ammo (and dual it if at all possible). When starting this fight, jump down to the lower platform and kill the Leapers without hitting the bombs. Even if they will not damage you, save them. When Theta appears, switch to the assault rifle and wait for him to walk near to a pack of those bombs. After that, blast the bombs with a quick burst of ammo. Keep luring him, not letting him get too close, and bomb him. Continue to do this until all the bombs are gone, then switch to your shotgun(s). Run directly at him, and buckshot any Leapers on your way. Run around him, blasting him with the shotgun. If it looks like he is about to hit you and you cannot get out of the way (as one hit from him will take out basically half your health), use the secondary function of the shotgun and back up while he is dazed from the double-barrel attack. Keep running around him and fire the shotgun until he falls. Note: If you need to reload your shotgun, perform the secondary attack then back up to reload. The stumble time will give you a little extra time to refill the slow-loading shotgun.