Theta was an experimental creature carrying the virus Dr.Cray mutegen realesed to stop the Illuminati to steal the virus. Delta haz team is the first to find it when they open a lift it. He has the ability to infect non-affected sentient beings, it is a carrier of the mutagen. Has the potential to effectively kill with one blow (depending on the difficulty)
Go into Infected mode and melee it for mutagen and launch parasites at it for health. This is the easiest way to beat the Theta.
theta - shoot the (censored) out of it with the pistol or Big Bouncy Gun or shot gun... anything works
super theta - shoot it with the Big Bouncy Gun, and when it unleashed the parasites, use the shot gun to eliminate the threat quickly and efficiently, then go back to the slaughter with the Big Bouncy Gun.