If you are lucky you have found an item that give you shock resistance or extra HP. Apart from that it might be good to have plenty of troops and chaff. If you are a demonologist it might take a while, since your summons are somewhat expensive. A necromancer could easily buy lots of goblin chaff and raise dead in some nice graveyard. A druid might get himself twoscore rabbits
Patience. But patience might run short when the pole starts to send spirits on you from afar.
Edit: I thought the lightning from the pole was 7 separate strikes. If it is a line attack you should focus on fewer and more powerful critters. The demonologist has plenty of these.