During battle can I play a one-shot card?
Yes, go right ahead. For most one-shots, the only requirement for playing them is that there's combat happening. As soon as it is, you can play your one-shots. You don't need to wait your turn or anything like that - there's no turn to wait for! Snoozers losers, you've got loot and levels to find.

From the original rules:


  Items – Using Them
  
  Any one-shot Item can by played during any combat,
  whether you have it in your hand or on the table. (Some
  one-shot Items, such as the Wishing Ring, may also be used
  outside of combat.)


There are no other constraints, and no "you can only play it when the other players say it's OK or are done talking."

As for Curses in particular, remember:


  Curse cards may be played on any player at any time. ANY time, do you hear me?