There is no skill that has an area of effect large enough to cover all 8 squares of the 2x4 configuration of enemies in the Cave of Ordeals 4. It will take you a minimum of two characters to clear CoO4.
Here are some skills with very large area of effect:
- Winged Slayer (Sword): (3x3)
- Dimension Slash (Sword): (1x5)
- Big Bang (Fist): (3x3)
- Magic (Any): Up to 3x3
CoO4 is still the best level to gain levels fast. The official Disgaea 2 Strategy guide by Doublejump books has a very long procedure for power leveling. Here are the high points:
- Make some level 1283 or higher Nekomatas by combining, capturing them, and repeating.
- Unlock the ability to create Carmillas.
- In the Dark World 5-2, kill the two Carmillas closest to the base, and the Geo Clone Symbol. Wait turns for the Level 1300 Carmilla to get to L2197. Kill 2 Level Up Geo Symbols, and wait 1 more turn. Attack the Carmilla to weaken her; hold her up during Round ends to avoid her gaining any more levels. Send your L1283 Nekomata back into the base panel and then throw in the Carmilla to capture her.
- Repeat the above step as necessary to get to Level 9999 Carmillas.
- Capture as many of the Level 9999 Carmillas as you want, and reincarnate them into Space Knights (boost your INT with all your bonus points).
- Max out an Armsmaster Specialist at 1900. Same for Manager (300), Mentor (300), Broker (300) and Professional (100). Also make a Firefighter, Aeronaut or Cryophile at 100 - whichever matches your Skull. Also make a maxxed out Lovers for both Magicians and Magic Knights.
- Also make 10 Level 19998 Nerds. (The guide notes that this is the tedious step.)
- Make a Skull in your favorite element (fire in this example). Equip the Skull with your best staff. Everything else should increase his SP. Put all of the Specialists above on his equipment.
- Teach you Skull Tera-fire, and then cast Tera-fire on an enemy immune to fire until Tera-fire is at level 52. This is best done on an Attack+X panel.
- Reincarnate the Skull into a Genius Space Knight. Switch out the Lover on the newly-minted Space Knight's equipment.
- Pass all Stronger Enemy bills in the Senate.
- Spend a couple of hours soloing the new Space Knight in Cave of Ordeals 4 to reach Level 9999.
(Seriously, that's the short version.)
At the final step, and every other step where non-monsters need to level up, CoO4 is the place to go.