“Different Dimension Dragon” cannot be destroyed by “Fissure”, “Mirror Force”, etc. Also, if “Different Dimension Dragon” is your only monster on your side of the field, your opponent cannot activate effects like “Fissure”, “Mirror Force”, “Raigeki”, etc.
“Different Dimension Dragon” can be destroyed while in your hand or Deck with “Chain Destruction”.
If “Different Dimension Dragon” is face-down, it can be destroyed by “Raigeki” and similar non-targeting cards.
Netrep Rulings:
If “Different Dimension Dragon” is the only monster on the field, monsters with 1900 ATK or lower do not attack directly.