public class TowerTargetJobSystem : ComponentSystem
{
    ComponentGroup enemyGroup;

    protected override void OnCreateManager()
    {
        enemyGroup = GetComponentGroup(typeof(EnemyData), typeof(Position));
    }

    protected override void OnUpdate()
    {
        if (enemyGroup.CalculateLength() == 0)
            return;

        var entityType = this.GetArchetypeChunkEntityType();
        var positionType = this.GetArchetypeChunkComponentType<Position>(true);
        var enemyChunks = this.enemyGroup.CreateArchetypeChunkArray(Allocator.TempJob);

        var job = new TowerTargetJob() { enemyChunks = enemyChunks, entityType = entityType, positionType = positionType };
        job.Schedule(this).Complete();

        enemyChunks.Dispose();
    }

    [BurstCompile]
    struct TowerTargetJob : IJobProcessComponentData<Tower, Position>
    {
        [ReadOnly] public NativeArray<ArchetypeChunk> enemyChunks;
        [ReadOnly] public ArchetypeChunkEntityType entityType;
        [ReadOnly] public ArchetypeChunkComponentType<Position> positionType;

        public void Execute(ref Tower tower, [ReadOnly] ref Position towerPosition)
        {
            var nearestEntity = default(Entity);
            var minDist = float.MaxValue;

            for (int i = 0; i < enemyChunks.Length; i++)
            {
                var chunk = enemyChunks[i];
                var enemyPositions = chunk.GetNativeArray(positionType);
                var enemyEntities = chunk.GetNativeArray(entityType);

                for (int j = 0; j < enemyPositions.Length; j++)
                {
                    var enemyPosition = enemyPositions[j];
                    var enemyEntity = enemyEntities[j];
                    var distsq = lengthsq(enemyPosition.Value - towerPosition.Value);

                    if (distsq < minDist)
                    {
                        minDist = distsq;
                        nearestEntity = enemyEntity;
                    }
                }
            }

            tower.Target = nearestEntity;
        }
    }
}