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 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. Try not to hit 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.