Happy Wars

They must be obtained in either Quick Match, Co-op or Special Games.
They cannot be obtained in story or Skirmish Mode.

You must finish the game to the end regardless of winning or losing. Disconnecting or quitting will forfeit any earned medals within the match.

More Kill Streaks
Claim 10 consecutive kills.
This is the same as Invisible, but you're allowed to die. The consecutive chain continues for a kill made within 10-15 seconds.

Mighty Warrior
Kill every enemy at least once.
This can be hard or easy depending on the number of opponents there are, and how tough they are individually. The easiest way to get this is in a 8v8 Co-op Match against a pushover team like Killer Tomatoes or Band of Clerics.

Halloween Assassin
25 kills as a single class without dying.
Can only be obtained once.
You'll get this along the way to NBK.

Special Forces
Destroy the Big Tower without losing any HP.
You'll want an Engineer with the Wall+ and Sacred Armor skills on your gear. Before taking down the tower, give yourself the armor to prevent damage, use Crash Time if you got it as a random skill, then rush to the tower, build a wall high enough so the bots can't walk up the side of the steps, and hope that you can take it down in time. Having others with you will help even more.

The Strongest Title
Destroy the Big Tower and kill all enemies in 1 game.
Are you a bad enough dude to become the Strongest Title? Using the Mighty Warrior medal as your base, the extra addition is to be the only one who takes down the tower. That's right. You. Alone. If anyone else interferes, it'll void the medal.

Ultra Happy
Over 150 hours of total play time.
Can only be obtained once.
Chances are if you've already finished all the X360 achievements, you'll be pretty close to this, if not already.

Halfway There!
384 total wins.
Can only be obtained once.

The Terminator
1000 total kills.
Can only be obtained once.

Golden Samurai
5000 total kills.
Can only be obtained once.

You Can't Win by Power Alone
Kill 500 Warrior types.
Can only be obtained once.

The Twilight of Magic
Kill 500 Mage types.
Can only be obtained once.

Pray in Hell
Kill 500 Cleric types.
Can only be obtained once.

Heart of Steel
500 total kills as any Warrior type.
Can only be obtained once.

The Devil
1000 total kills as any Mage type.
Can only be obtained once.

Doctor
6000 total heals as a Cleric.
Can only be obtained once.

Omaha Beach
300 kills using the ballista.
Can only be obtained once.
Really easy to boost in Castle Defense.

Medic!
Used Resurrect 500 total times.
Can only be obtained once.

Pop-Up Ace
Appear on Hero Pop-up 100 times.
Can only be obtained once.

Hat Trick
Destroy the Big Tower 500 times.
Can only be obtained once.

Interrupting Cow
Interrupted enemy's Team Skills 1000 times total!
Can only be obtained once.
Don't even try going for this. Trust me, you'll thank me later.

Desert Fox
Be in the Castle 200 times when your team destroys the Big Tower. (Desert or Zodiac stages)
Can only be obtained once.
Being dead inside the castle once the tower is down still counts for this and the related medals.

Seawolf
Be in the Castle 200 times when your team destroys the Big Tower. (Coral or Deep Sea stage)
Can only be obtained once.

Dragon Blood
Be in the Castle 200 times when your team destroys the Big Tower. (Rainbow or Snowman stage)
Can only be obtained once.

Demon Lord
Be in the Castle 200 times when your team destroys the Big Tower. (Twilight or Pumpkin stage)
Can only be obtained once.

Heavenly Giant
Be in the Castle 200 times when your team destroys the Big Tower. (Bird God or Sunset stage)
Can only be obtained once.

Coup de Grace
Kill the boss in a Special Challenge.
Will have more info on this late