I added Houndoom because its ability to pursuit and kill ghosts and celebi, it's fast, it can burn, it can beat common sweepers like superachi/celebi...
About the descriptions, some suggestions:
Blissey => You should at least mention status (toxic/t-wave)
Celebi => You forgot Perish Song. I would also mention the bulky sweeper set (CM + seed)
Forretress => The other big difference with Skarm is that Skarm can phaze. You mention Houndoom as a pursuiter, but Tyranitar is usually better doing that.
Gengar => You forgot Hypnosis and Perish Trap.
Tyranitar => I would mention its best sets: T-tar is the best late game sweeper with DD, and one of the best hard hitters of the game with a choice band. It can also beat stalls with a mixed set with Sub, or Pursuit ghosts to death.