Can this be used on more then one group?

Or once all figures in the group are defeated can it be assigned to another group of figures?

No and no. It's assigned to one group, however as soon as it's used, it's depleted (meaning removed for the remainder of the game). So if it's assigned to a group with more that 1 figure, as soon as 1 figure uses it, then the "last resort" card is gone and can't be used again. So the best way to currently use this, would be to add it to a Nexu, Then you could attack 12 spaces rather the 8 you would get with a standard 1x1 sized unit.