As a mental exercise, I created a party of 30th level characters that was able to automatically win initiative against Orcus and (provided they started within a reasonable distance of him) was able to keep him stunned and completely unable to act while they brought his hit points down to 0, provided nobody rolled a 1 on an attack roll. On average, this would take about five rounds.
In the course of this exercise, I discovered how useful the following powers are in boosting your or your party's chance of hitting:
1. Lead the Attack (Warlord Daily 1): One Strength attack vs. AC. If you hit, allies within 5 squares gain a power bonus to attack rolls equal to your Intelligence modifier +1 against the target you hit until the end of the encounter.
2. Feinting Flurry (Rogue Daily 19): All the target's defenses against your attacks take a penalty equal to your Charisma modifier. This is an Effect, so it does not require you to actually hit with your attack, and can be sustained as a minor action.
3. Hand of the Gods (Paladin Encounter 17): Allies adjacent to you gain a power bonus to attack rolls equal to your Wisdom modifier until the end of your next turn. This is an Effect.
4. Armor Splinter (Ranger Encounter 13): Make two Strength attacks vs. AC. If one attack hits, the target takes a penalty to AC equal to your Wisdom modifier until the end of your next turn. If both attacks hit, the penalty is equal to your Wisdom modifier +2.
5. Plague of Doom (Cleric Encounter 13): Wisdom attack vs. Fortitude. If you hit, the target takes a penalty to all defenses equal to your Charisma modifier until the end of your next turn.
6. Chains of Sorrow (Fighter Encounter 13): Strength attack vs. AC. If you hit and are wielding a flail, the target takes a penalty to all defenses equal to your Dexterity modifier until the end of your next turn.
Essentially, the Rogue/Shadow Assassin/Demigod (multiclass Fighter) wins initiative, uses an action point to gain +8 to his attack rolls (+4 from his paragon path ability, +4 from the tactical Warlord), uses Feinting Flurry (lowering Orcus's defenses by -8 against his attacks) and stuns Orcus until the end of his next turn with Perfect Strike (easily hitting Orcus's AC, Fort and Reflex).
The Paladin/Champion of Order/Demigod (multiclass Ranger) activates Hand of the Gods to give a +9 power bonus to the attack rolls of the Ranger/Pit Fighter/Demigod (multiclass Fighter) and the tactical Warlord/Kensei/Demigod (multiclass Fighter). The Ranger spends an action point (gaining a +4 to attack rolls from the tactical Warlord) and uses Armor Splinter, most likely hitting with both attacks and lowering Orcus's AC by 11. This, together with the Paladin's power bonus to attack rolls, makes it almost certain that the Warlord will hit with Lead the Attack, giving everyone a +10 power bonus to hit Orcus until the end of the encounter.
In the next round, the Paladin uses Armor Splinter again to keep Orcus's AC low, and the round after that, the Rogue uses Chains of Sorrow to lower ALL of Orcus's defenses by 9 (between Feinting Flurry, Armor Splinter, and Lead the Attack, it doesn't matter that his Strength is rather low).
In the fourth round, the Cleric/Radiant Servant/Demigod (multiclass Rogue), who would have already used Feinting Flurry in a previous round, uses Plague of Doom to lower all of Orcus's defenses by 9.
By this point, some of the PCs would have started running out of encounter powers, and so they can use Divine Miracle to refresh Armor Splinter/Chains of Sorrow/Plague of Doom if Orcus is still alive.
