A keen eye is a must, especially when it comes to solving the game's puzzles. While you may have a limited number of parasite energy points, you get more with each successful battle. P.E. helps you conserve ammo, and you won't have to deal with reloading your gun. You should also use the P.E.'s area of effect. Narrow areas are good for taking out distanced opponents.
Concentrated areas are great for toasting tightly knit groups.
Using your powers could mean the difference between staying alive and pushing up daisies. And the only way to earn bounty points is by engaging the enemy. While you might want to get through your objectives as quickly as possible, you'll be doing yourself a favor by passing through infested areas and doing some spring-cleaning. Check your map periodically. Areas colored red are full of enemies waiting to eat lead.