You have a couple of choices:
1. Kill them fast before they can stack a lot of poison on you. You don't need to be able to 1 shot them. They only poison once every other turn at minimum
2. Get a couple of Cleansing Rags attached to a weapon so you can remove poison each attack, and combine that with some heals/regen through either a beginner wand or from heart gemstone or vampirism
Generally the main goal is just... get rid of the poison dealers first. The longer they live, the more poison will stack on you. All other damage is manageable.