Gruenhilda was tasked to instruct Kevin Masterson how to properly wield the Thunderstrike mace by the Asgardian goddesses Sif and Hecara after Steve Rogers gave it to the boy; the Asgardians felt Kevin's father was owed a debt of gratitude for his courage and honor by first merging with, then briefly replacing Thor (Thor Odinson) after the thunder god was banished. When Thor returned, Odin created the Thunderstrike mace, gave it to Kevin's father, and he became the hero Thunderstrike (Eric Masterson) whose final act was to sacrifice himself to save the world. Now that the mantle had passed from father to son, the debt could be repaid.
On Earth, Gruenhilda found Kevin just as he was attacked by the Spartans strike force of the sports mogul Adam Mann. They were trying to secure the mace for their employer who sought to steal its power. Although a valiant warrior, Gruenhilda could not prevent them from taking the mace, but she saved Kevin's life and the life of his adoptive-father, Bobby Steele. Staying with Kevin in an adopted civilian identity of Hilda Grune, they infiltrated Mann's home to retrieve the mace, and Gruenhilda led the attack against Mangog after the demon was accidentally summoned through Mann's Mysticator; a machine meant to supply him with God Force by siphoning the power of ancient artifacts. Gruenhilda was not powerful enough to defeat the beast, but she saved Kevin from being devoured by it. Fighting side-by-side with the Avengers to prevent Mangog from destroying New York City, she helped Kevin (as Thunderstrike) and Thor exile the creature elsewhere. Afterward, Gruenhilda was with Kevin as he challenged the Rhino (Aleksei Sytsevich) to a rematch.