Geddoe is beatable in Heavy. Heavily resist thunder and he's in trouble. Immune magic and he's likely losing. Try to outtank him and... well, good luck. Zemeckis tried that to no avail the last time the leader of the Southern Frontier Defense Force was in. Tsarvina Alena is even stronger and faster than Zemeckis, but not quite as tough. She'll come at Geddoe, claws flying and trying to rip him apart with a critical hit. Parry and repel slow down Alena's offense, leaving her in a vulnerable position. Geddoe can hurt the princess badly with a single Hammer of Raijin, and finish her off with any of his other spells from his True Lightning Rune. All of Alena's toughness won't do her any good here, as she has no defense against the True Lighting Rune's overwhelming power.
