I so have to disagree. Ielena is awesome, she really is. Sure, Stop Opp Ability hurts, but with Nightmare you should really have no problem to play around it. Seriously.
With Sheitane first turn and Kenny and Ielena still sitting on your hand your opponent should get multiple migraines thinking about how to stop all of these. Just imagine you would have someone like Gina Glitt or Uranus on your hand as well. Sure it's constructed, but it demonstrates that Nightmare can easily outplay Stop Opp Ability if done right.
Yes, Ielena is a 5 star card, but she comes with the important 8 Power to take down cards like Lamar (well, pretty much every All Star actually), Hikiyousan (even though he doesn't see much play) and other Ulu Watu or Dorian. Even though damage reducing hurts, it only hurts half as much as against other cards. Example: Zatman defeats GraksmxxT and deals 1 damage. Ielena defeats GraksmxxT and deals 1 damage while you gain 1 life. This can be enough already.
Oh, and when it comes to the all mighty Pussycats, Ielena and Kenny defeat them on their own already. These 2 together with Sheitane and her poison are Pussycats well... err... Nightmare if played right.
After all, Ielena can deal a devastating 10 life difference blow when she furies, just like as Kenny can win games with his 15 life difference fury. And as long as you are willing to sacrifice them if you have to, your opponent has the trouble of dealing with them.
And don't underestimate Dieter.