This is just a simple solution to the game https://node-defender.herokuapp.com/

node-defender.js

var enemy, attackModes, positions, types, mostPos, closestPos;
 
positions = {};
types = {};
enemy = _.first(roundInfo.getMobs()); 
commander.target(enemy.id);
commander.attackMode('power');
 
_.each(roundInfo.getMobs(), function(mob, idx){
    if (!_.has(positions, mob.position)) positions[mob.position] = [];
    positions[mob.position].push(mob);
 
    if (!_.has(types, mob.type)) types[mob.type] = [];
    types[mob.type].push(mob);
});
 
mostPos = _.max(_.pairs(positions), function(elem){
    return elem[1].length;
})[0];
 
closestPos = _.min(_.keys(positions));
commander.target(positions[closestPos][0].id);
 
if (closestPos >= 2){
    commander.attackMode('ranged');
    if (_.has(types, "flyer")){
        var furthest = _.first(types.flyer);
        _.each(types.flyer, function(mob, idx){
            if (mob.position > furthest.position) furthest = mob;
        });
        
        commander.target(furthest.id);
    }
}
 
if (positions[mostPos].length >= 3){
    commander.target(positions[mostPos][0].id);
    commander.attackMode('collateral');
}
 
if (_.has(positions, 0)){
    _.each(positions[0], function(mob, idx){
        if (mob.type == "grunt" || mob.type == "bruiser" || mob.type == "trooper"){
            commander.target(mob.id);
            commander.attackMode('power');
        }
    });
}