Spec and abilities:http://gw2skills.net/editor/?fQAQNBmQDbkRrdvYzcjfPRI94ZXjXQPc/ucGVB3B
The skills that you choose in Spite and Death Magic are interchangeable, except maybe II and V in Death Magic.
The job as I see it: stay alive, cc, and collect bags. Staying alive is important not just for selfish reasons, but also because you can be there to revive. CC is to slow down and debuff targets, while making large groups scatter and/or run away. Collect many many bags because you can hit so many targets at once.
-Staff is for nearly all offensive operations. The staff is like a torch when you're surrounded by zombies...for some reason people hesitate to cross the marks. 9 times out of 10 they set them off and will run back/away from you. The staff is, like you say with ele, also perfect for keeping keep defenders away from the walls while your team gets the door down.
-Dagger/Warhorn is for defense and running around. Skill 5 gives a speed boost (I also switch out Corrosive Poison Cloud for Signet of the Locust if I have to run a good distance, but SOL is kind of useless in combat). If you're outnumbered and need to run away, lay down some marks if you can, and switch to DW and hit 5, and runrunrun...you go faster and they go slower. Another cool thing about the warhorn is that you can use Locust Swarm during a melee fight (the dagger autoattack is strong) and it's basically a DOT while you're standing near your target.
-Healing/staying alive: people want well of blood, but in all honesty, too bad. The guardians and eles should be putting down the heals/fields, and Consume Conditions is what will keep you alive more efficiently. I use Death Shroud when I'm about to die---this is why I use those last 10 points in Soul Reaping, with skill II making DS last longer. Another great "oh shit" button is Plague. For example, if I'm caught running alone by 5 or 10 people, I pop plague and they nearly always seem to scatter, before I even do any moves. It's also great for big fights. The only time I die while in Plague form is when there is heavy incoming AOE damage. Finally, the best way to stay alive is to stick close to a good commander and follow directions (unless they tell you to put on well of blood :))
-Corrosive poison gas cloud: I've tried the rest, and keep coming back to this. Those debuffs are huge and it's a combo field. Place it at a choke point when an enemy group is approaching, and place it on your group (which is hopefully stacked) during big fights.
-Epidemic: This is the fun part. Once I've got the gas cloud down, I hit Signet of Spite and then Epidemic, and then I sit back and watch the numbers fly. Epidemic can be tricky because oftentimes my target will die before I can cast the spell, but when it works it's so worth it.
-Why a Power/Toughness build when you're inflicting conditions? The conditions are there to tag targets. Most people shed their conditions somehow, so I don't want to rely on that. I like the power, maybe it's just psychological, but when I had a condition build it simply did not feel as effective. This is the build that keeps me alive and helps me escape in situations where other builds/professions did not.
That's all I can think of for now. I'm wondering, how is it that you're floundering? Die too fast/often, or? If I didn't answer your question above, maybe I can if I know where you need help specifically.