20 Droideka's are very tough to defeat... seeing as most Jedi are cautious around 1 or 2 of them (most back away as has already been pointed out)... Kenobi's Soresu would aid him... but alas I think he would fall in such a battle... although he would take a few with him...
Then again if he faces them 1 or 2 at a time then Kenobi wins.