"Berserk Dragon" cannot be Normal Summoned or Set, and cannot be Special Summoned EXCEPT by the effect of "A Deal with Dark Ruler". So "Berserk Dragon" cannot be Special Summoned with "Monster Reborn", etc., even if Special Summoned properly first.
If "Berserk Dragon" is removed from play with "Interdimension Matter Transporter", you cannot try to resolve "Berserk Dragon"'s effect before it returns to play; you must return it to play first and then it loses 500 ATK.
While "Skill Drain" is active, "Berserk Dragon"'s effects are negated. But "Berserk Dragon"'s effect doesn't count turns, so if "Skill Drain" was active for 4 turns, and is then destroyed, do not reduce "Berserk Dragon"'s ATK for those 4 turns.