Check out jrichard's Untwinked Lightning Trapper Guide. Make sure to use your one point wonders and you'll do great. Fireblast is a very good backup for your traps. You may want to consider putting only one point into Shadow Master and putting the recommended points for SM in synergies instead.
Edit: Oh, I should mantion that the sentry/fireblast trapper is one of my favorite characters. You can take her pretty much anywhere in the game(Ubers excluded) solo even when your gear isn't great.