Yellow jacket is Hank Pym, a scientist whose discovery of what he named Pym particles lead to him becoming a size-changing superhero under the title Ant-Man. Some time later, he took on the title of Yellow-jacket. During the Civil War, he sides with pro-registration forces and fights Anti-registration forces attacking a S.H.I.E.L.D. convoy. To beat him you need characters with powerful attacks and be able to perform at least 2 different targeted fusions. The second one is in case a character dies and cancels one out. Wait until he attacks and jump when he does, then attack his arms. When you see his health bar turn grey use a targeted fusion on what ever body part is on the platform you are on.