HGNC gene mart... I assume you meant Entrez Ids to Gene Symbols.

http://biomart.genenames.org/martform/#!/default/HGNC?datasets=hgnc_gene_mart

Orelse you might find information in the following file (For all species),

ftp://ftp.ncbi.nlm.nih.gov/gene/DATA/gene_info.gz

===========================================================================

gene_info recalculated daily

       tab-delimited
       one line per GeneID
       Column header line is the first line in the file.
       Note: subsets of gene_info are available in the DATA/GENE_INFO

directory (described later)

tax_id: the unique identifier provided by NCBI Taxonomy for the species or strain/isolate

GeneID: the unique identifier for a gene ASN1: geneid

Symbol: the default symbol for the gene ASN1: gene->locus

Synonyms: bar-delimited set of unofficial symbols for the gene

Complete information about the file contents

ftp://ftp.ncbi.nlm.nih.gov/gene/DATA/

Species info:

ftp://ftp.ncbi.nlm.nih.gov/gene/DATA/README_ensembl

Have you tried batch queries on the NCBI site ?