Luke is most certainly nothing close to being trained. Apart from the obvious knowledge of the Force which he lacks, he lacks common sense as well (throwing his lightsaber away while facing SIDIOUS of all ppl. Remember what Obi Wan said to Anakin in AOTC: "This weapon is your LIFE" This is especially true for someone with a little over only a Younglings level of training) Obi Wan in TPM was superior to Luke by far, and he was still a padawan. Do you honestly think Luke could have stood up to someone like Darth Maul and survived? He only won against Vader because 1) Vader is weak, conflicted and overall a tragic and pathetic figure. 2) Vader didn't want to kill his son, because he was holding out of him converting to his side, and maybe some feelings of love and affection 3)Palpatine POSSIBLY may have told Vader to take it easy on him so that he can convert him to the dark side. Kind of like Anakin vs Dooku, where Palps played Tyrannus...here he may be attempting the same thing with Vader and Luke. Anyhow, defeating Vader is no biggie. Obi Wan could have done it in ANH had it not been for the fact that he wanted Luke to escape and not stand and watch the duel, and the thousands of Troopers who would have shot and killed them all had Ben killed Vader. Also, the only reason Sidious never got anyone apart from Vader after ROTS is that no other Jedi who survived Order 66 would be stupid enough to join Darth Sidious. Even if they did pretend to join, it would only be so they could get close and attempt to kill him. However in Luke Sidious sensed a kindred soul to Anakin, a conflicted and isolated lad who would be easy to mold. Unfortunately, Sidious underestimated the heart of Luke, and his ability to awaken the Jedi within Vader.