Ok, this is actually easier than it should be. If you want to obtain Lu Bu very early on, then do this. Play as Cao Cao. For your first objective, you are given a choice of eliminating Yuan Shu or Lu Bu. Go eliminate Yuan Shu. Then, your next objective will be to eliminate Lu Bu, and you have 20 turns! It is so easy. Once you do this, Lu Bu will surrender, and he will join your party! See how easy?! He starts off at Level 6, with an attack of 71! Give him the Avenger, and he has an attack of 86! Plus his skill is Menace already, so you can give somebody else the "Crescent", which will give them Menace. Wow is he amazing!