Disclaimer: these characters are owned by Lucasfilm, not me.
Rex falls to the ground as Fives pulls the trigger. It's an executioner's shot to the head. Unfortunately, it's too late. Ahsoka lies against the Corascanti duracrete with a blaster burn so bad, Fives knows that all the Bacta in the Republic won't save her. For all of Five's heroic actions, her death is mundane and anticlimactic.
Fives picks up his blaster and shoots Rex through the front, thankful that the EM field coming from the broken Seperatist ship wreckage scrambles the HUD screen in his helmet to the nine Corellian Hells and back.
With a soft Mandelorian curse slipping through his lips, Fives blasts the side of his leg at an awkward angle, making it look like rebound from a light saber. Like he hopes, it drops him to his knees in pain, grazing a major nerve. Picking up Ahsoka's light saber, he cringes, then cuts off his hand, watching in fascination as the saber instantly cauterizes the wound. He places both weapons next to Ahsoka with his left hand, making her grip the blaster loosely with one hand. Her head rolls to the side and he can hear faint breathing getting slowly fainter.
The world tips and turns as Fives falls to the ground, passing out from pain. The adrenaline has purged and left his system. Fives wakes eight days later with a shiny new hand, in a new empire.