Class, build, Difficulty Level, Character Level?
Scoring as many points as possible in the debate will help you gain allies in the fight (see Wiki for exact details).
The Parasite is an Outside, so Banishment weapons do extra damage ( e.g. Adamantite
Using top tier potions/scrolls/etc will help. Even if a summons only last a few seconds, it can give you breathing room. And if it isn't attacked, the summons damage can add up over a long fight.
Potions of Lessor Restoration and Scrolls of Circle of Restoration will aid greatly.
It's a tough fight but doable.