My understanding is that a 'completed' task is one in which you fulfill the roll requirements and thus can roll the remaining dice without discarding one as a penalty. If I use a spell to defeat a monster after I roll, do I then need to also complete an additional task in order to re-roll without discarding a die? This is how the iOS game "Elder Sign: Omens" works, but I don't see any indication in the rules of it. I'm looking for documentation that implies this is how it should play.
This is a quote from the Elder Sign rulebook, as currently posted online. Page 8.
Failing Rolls If the player is unable or unwilling to use his die results to satisfy any task’s requirements, the roll is considered a failed roll. He must discard one die of his choice from the dice pool and roll the remaining dice in another attempt to complete a task.
There is no other errata or clarifications or asides that alter this rule (other than a brief aside to confirm that yes, you can choose to intentionally fail rather than spend too many dice on one task). It doesn't matter that you managed to complete a task on the adventure card in some other way, in your case by killing a monster with a spell. The roll still failed. You must still discard a die.