Which certainly works, though I would recommend using the NPC creation rules on DMG page 187. Only really takes a couple minutes longer, but less need to make any real guesswork or arbitrary decisions, compared to using the rules for designing your own monsters.

Step One: Choose Level (20)

Step Two: Choose Race and Class (Human Fighter)

Check the NPC listings on page 188 for what class features you get: NPC Fighters get Combat Challenge, so they can mark enemies, and take an attack as an Immediate Interrupt if their mark shifts or attacks someone other than them.

Step Three: Determine Ability Scores

Start with the standard array, resulting in:

Str: 16, Con: 12, Dex: 14, Int: 11, Wis: 13, Cha: 10

Apply your Human bonus to Str, and then add in level bonuses:

Str: 23, Con: 13, Dex: 19, Int: 12, Wis: 14, Cha: 11

Step Four: Determine Hit Points and Healing Surges:

Fighter NPCs have 8 per level, plus Con score, so: 173 hp.

As a paragon tier NPC, the character has 2 Healing Surges.

Note that, as an NPC, it can use Second Wind once per combat.

Step Five: Calculate Defenses

AC and Defenses are calculated using the normal formula (10 + 1/2 level + relevant ability modifier), with armor or shields added in the usual fashion. Then it gains an NPC level bonus to represent various extra bonuses, magical or otherwise. For a 20th level character, this bonus is +7. Finally, add in any racial or class bonuses. Thus:

AC: 10 (base) + 10 (1/2 level) + 7 (Scale) + 7 (NPC bonus) = AC 34

Fort: 10 (base) + 10 (1/2 level) + 6 (Str) + 7 (NPC) + 1 (Human) + 2 (Fighter) = Fortitude 36

Ref: 10 (base) + 10 (1/2 level) + 4 (Dex) + 7 (NPC) + 1 (Human) = Reflex 32

Will: 10 (base) + 10 (1/2 level) + 2 (Wis) + 7 (NPC) + 1 (Human) = Will 30

Step Six: Choose Powers

At Will Powers: Choose One (Reaping Strike). Choose a second, due to being Human (Cleave)

Encounter Powers: Choose one of the NPC's level, and a second one (at a lower level) if it is 11th level or higher. (Exacting Strike, Anvil of Doom)

Daily Powers: Choose one of the NPC's level, and a second one (at a lower level) if it is 21st level or higher. (Reaving Strike)

Utility Powers: Choose one of the NPC's level, and a second one (at a lower level) if it is 11th level or higher, and another if it is 21st level or higher. (Iron Warrior, Into the Fray)

Iron Warrior: Daily Power, Minor Action, Spend a Healing Surge to regain 2d6+44 hitpoints and make a save against one (save ends) effect.

Into the Fray: Encounter Power, Minor Action, Move 3 squares, as long as you end adjacent to an enemy.

Step Seven: Choose Skills

Fighter NPCs can choose two skills from the Fighter class list. Choose a third, due to being Human. (Athletics, Endurance, Heal.)

Athletics: 10 (1/2 level) + 5 (Trained) + 6 (Str) = +21

Endurance: 10 (1/2 level) + 5 (Trained) + 1 (Con) + 16

Heal: 10 (1/2 level) + 5 (Trained) + 2 (Wis) = +17

Step Eight: Choose Equipment

Maul, Scale Armor

Step Nine: Calculate Attack and Damage Bonuses

Calculate bonuses in the normal method, plus the NPC bonus as mentioned in Step Five.

Basic Attack with Maul: 10 (1/2 level) + 6 (Str) + 7 (NPC) + 2 (Profiency) = +25 vs AC. Damage: 2d6 + 6 (Str) + 7 (NPC) = 2d6+13.

Running similar numbers for our powers, we get the following:

Reaping Strike: +25 vs AC: 2d6+13. *Miss*: 6 damage.

Cleave: +25 vs AC: 2d6+13, and a different adjacent enemy takes 6 damage.

Exacting Strike: +31 vs AC: 4d6+13 damage.

Anvil of Doom: +25 vs AC: 4d6+13 damage, and the target is stunned until the end of your next turn.

Reaving Strike (Reliable): +25 vs AC: 10d6+13, and push the target 1 square.

Step Ten: Choose Rituals

Not needed for this character.

NPC generation: Complete! Total Time: 10-15 minutes. Completely by the books, and resulting in a simple and easy to run NPC, that still feels and functions completely like a Fighter.