Obi-Wan is not there to defeat Anakin, he is there to defeat the Vader aspect of Anakin's personality.
Consider that Obi-Wan's goal here is to goad Darth Vader into defeating him instead of coming in contact with Luke before Luke has obtained more Jedi training from Yoda.
And what better way to put your opponent into the mindset that they are in control of the situation than by pretending to be afraid, or making wild claims about becoming more powerful? Or cheekily referring to him by only using part of his name/title?
Obi-Wan needs to get Darth Vader off balance enough for Vader to attack him without thinking and defeat him - as a distraction for Luke to get away. Can you imagine what would have happened if Vader had time to think about the confrontation, and captured Obi-Wan instead? Vader could have used Obi-Wan as bait to reel Luke back in. Obi-Wan must of foreseen this and decided to take this seemingly futile path (but which led to eventual success).
By referring to Anakin as "Darth", and feebly claiming that he (Darth Vader) can't win, Obi-Wan places himself in a position as a-soon-to-be-be-defeated adversary. Perhaps Obi-Wan was bluffing about becoming "more powerful that you (Darth Vader) can ever imagine" (after all, all he becomes is a voice in Luke's head - right?)
But it's not until later that the audience and Vader realize that Obi-Wan wasn't talking about their confrontation (Darth Vader & Obi-Wan's final battle) when he was referring to Darth's (Vader's) ultimate lack of "win".
Obi-Wan's "win" is the ultimate reconciliation of Anakin and his son, Luke. Which has the side effect of re-balancing the force, (or do I have that the other way around?)
Darth Vader has to be taken out, so Obi-Wan sacrifices himself so that the confrontation between Luke and his father is delayed until later, after Luke has a more complete perspective of their actual relationship. Darth Vader isn't taken out by the lightsaber battle - he seemingly wins.
But the way Obi-Wan disappears upon his defeat plants a seed of doubt in Vader side of Anakin/Vader, which eventually grows to the point that Anakin is able to resist the Vader persona and confront the emperor at the crucial time.