Play Stage 7 (See Kallen Off > Japan Route) and do not attack Lorelei. Convince her, using Shotaro piloting Tetsujin 28. After defeating all other enemies in Stage 7, Lorelei will challenge Kouji to a duel. You can now defeat Lorelei with Kouji piloting Mazinger Z. If done correctly, Lorelei will run away when defeated. She will rejoin you during Stage 32 (Earth Route). Note: You must take the Earth Route after Stage 31. Also Lorelei cannot be in the same party as Anew.