EDIT: Also see this question which expands on the code below, adding tests for whether objects are behind cover.

If your enemies have colliders attached, you can use Physics.OverlapSphere to quickly get your initial list of enemies within a certain radius.

You can then loop through each enemy within this list and determine the relative distance to the player, and apply some level of damage to each enemy within the radius, proportional to its distance.

It would end up looking something like this. It assumes that your enemy objects have both a collider and a script called "Enemy" attached.

In C#:

void AreaDamageEnemies(Vector3 location, float radius, float damage)
{
    Collider[] objectsInRange = Physics.OverlapSphere(location, radius);
    foreach (Collider col in objectsInRange)
    {
        Enemy enemy = col.GetComponent<Enemy>();
        if (enemy != null)
        {
            // linear falloff of effect
            float proximity = (location - enemy.transform.position).magnitude;
            float effect = 1 - (proximity / radius);

            enemy.ApplyDamage(damage * effect);
        }
    }
}

And in Javascript:

function AreaDamageEnemies(location : Vector3, radius : float , damage : float )
{
    var objectsInRange : Collider[] = Physics.OverlapSphere(location, radius);
    for (var col : Collider in objectsInRange)
    {
        var enemy : Enemy = col.GetComponent(Enemy);
        if (enemy != null)
        {
            // linear falloff of effect
            var proximity : float = (location - enemy.transform.position).magnitude;
            var effect : float = 1 - (proximity / radius);

            enemy.ApplyDamage(damage * effect);
        }
    }
}

(untested code, for illustration only!)

Take a look at the reference for the built-in function AddExplosionForce (with JS example, uses also Physics.OverlapSphere).