One-Armed can only carry 1 weapon, but with a high attack speed. So focus items that make you tankier, and boosting the damage output of the attacks you do have. I started the run with the Ghost Axe, for the +1% damage every 10 kills, at around round 14+ I swapped it out for the Nuclear Launcher with the carried over attack increase which was about +200 at that point. Any kind of Explosive weapon will be good to deal with the hordes of enemies for the rest of the run.