Of all the passages used to defend the traditional view of final punishment, one stands out as by far the most difficult for the conditionalist. ((Some would point to Revelation 14:9-11 as being equally or more difficult. However, this is only true if one is not aware of Isaiah 34:9-10. Revelation 14:9-11 doesn’t actually say anyone is eternally tormented. Rather it is inferred that smoke rising forever means that the fire burns forever and thus everyone being burned is eternally tormented in an ever-burning fire. However, Isaiah 34:10 uses the idiom of smoke rising forever to speak of the destruction of a city, not of anyone or anything actually burning and producing smoke. The resources in the footnotes for Revelation 20:10 can also give more explanation of this passage.)) As you might imagine, I don’t consider the challenge to be insurmountable. However, it is a challenge. This is the one passage in the entire Bible that actually says, on its face, that anyone will be tormented for eternity. ((Some passages mention torment (e.g. Luke 16:19-31), some mention eternity (e.g. Matthew 25:46), and some say things that one who has been told from childhood that hell is a place of eternal torment will understandably assume is referring to eternal torment (e.g. Mark 9:48). Revelation 20:10, however, actually outright says, of the devil, beast, and false prophet, that “they will be tormented day and night for ever and ever.”))
The explanation I would give, which many other conditionalists would give (in varying forms), is itself simple: John sees a vision where three beings are thrown into a lake of fire to be tormented for ever and ever, but the vision itself symbolizes the destruction of the things the images represent in real life.