It's possible that she had Seven bend down, like to pick something up. That way, she could easily swing the axe down on his head. Also, she was hiding it behind her back, if you remember, so she could have just found a moment where she and Seven were alone to pull it out quietly and kill him. With him gone, she could kill Santa and June much easier. Even if Clover is small, I think she could still hack and slash Santa to death (or maybe he sacrificed himself for June) and yeah, June is an easy kill.