Along with the new loot, there are now 4 added achievements for fighting in Mafia Wars.
Ice to Meet You - Ice 20 enemies
Everybody Chill - Ice 100 enemies
Ice, Ice, Baby - Ice 500 enemies
Kick Some Ice - Ice 1000 enemies
So, all that you have to do is fight really. To ice an opponent, you just knock their HP under 20. If someone is already below 20 HP they cannot be iced until they recover some health.