Trigger based AOE targets

// This is the list of conditions for choosing targets when you enumerate them
// if it returns false the unit is not enumerated.  I used bj_lastReplacedUnit, 
// as the source of the spell, you can use whatever global you want there.
function TargetConditions takes nothing returns boolean

// This checks to see if the unit is an ally of whichever unit you assigned to
// your global that represents the spell's source
    if(IsUnitAlly(GetFilterUnit(), GetOwningPlayer(bj_lastReplacedUnit)))then
        return false
    endif

// This checks to see if the unit is immune to magic
    if(IsUnitType(GetFilterUnit(), UNIT_TYPE_MAGIC_IMMUNE))then
        return false
    endif

    return true

endfunction

// This function actually does the damage; I just used a ForGroup iteration
// Another popular method is a FirstOfGroup iteration, but this should work fine.
// The difference is that ForGroup requires a callback while the FirstOfGroup
// looping can be done inside a function.
function DamageEnumUnit takes nothing returns nothing
    local unit u = GetEnumUnit()

    call UnitDamageTarget(bj_lastReplacedUnit, u, [Damage amount], ATTACK_TYPE_MAGIC, DAMAGE_TYPE_NORMAL, null)

    set u = null
endfunction

// This is where you will enumerate the group.  Probably a main function.
function Stuff takes nothing returns nothing
    local boolexpr bx = Condition(function TargetConditions)
    local group g = CreateGroup()
    local real x = [The x value at the center of your area]
    local real y = [The y value at the center of your area]
    local real range = [The radius of the circle in which units are enumerated]

    set bj_lastReplacedUnit = [The source of the spell or damage]
    call GroupEnumUnitsInRange( x, y, range, bx)
    call ForGroup(g, function DamageEnumUnit)

// Cleaning Up the Handles
    call DestroyBoolExpr(bx)
    call DestroyGroup(g)
    set g = null
    set bx = null
endfunction