You can either 'cash out' and forfeit your kill streak - or you can continue your challenge, continue your kill streak, but if you die you lose the entirety of your loot.
Enrage you can choose what enrage you start at, so there's no real thing as 'resetting enrage'. You can choose the enrage you start a fresh fight (e.g: no kill streak fight) at, between 0 and the highest enrage you have successfully killed.
Unslain Bosses - Clan