If combat has started and the other players start discussing what to do to me can I play a one-shot card while they are discussing?
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:
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:
Also, don't forget you don't have to wait all that long for them to decide what to do, version 1.6 clearly states:
So go ahead and kill it while they discuss, you'll either force them into making a mistake, or get the treasure ;)