For a level 20 Magus you're looking at base +12 Will, +3 or +4 from Wisdom, probably +5 from magic item(s), +1 permanent resistance, and probably 5+ in other bonuses I can't think of, assuming your game isn't Mythic in which case a spell level 5 scroll wont do anything anyway. This puts his bonuses somewhere around 20-25.
So you have as little as 5% (nat 1) or (optimistically) as much as 20% chance that he will fail. For every method you use to reduce his roll by 1, you increase that by 5%. It would be nearly impossible to reduce his roll to the point that you have a 95% chance of his failure.
While the dark wanderer seems to be in ridiculously meta mega-high fantasy games, the numbers I'm using assume normal level 20's using an average of the level 20 character gen gold.
Your flat footed problem is easier to solve by methods provided by MrLemon, I would likely focus on that opportunity against a caster type and use whatever your scroll is on a less Willful opponent. Other methods of denying Dex (and also Kensai's Int) to AC (AKA allowing sneak attacks) include:
-TWF and the Feint feat tree up to Improved Two-Weapon Feint if your GM allows you to study re-train feats. You lose one attack in your full attack on a Bluff check, and continue the rest of your 5ish swings.
-Flanking. Hire a legendary fighter or monk to stand on the other side of your foe and Greater Grapple them every opportunity every turn. Alternately, have your buddy trip them. While prone, you are susceptible to sneak attacks. Magi wont have the greatest CMD's.
Finally, with the Blackblade Kensai, you should be more worried about his ability to enchant his weapon and full attack you. His black blade will likely have an 18 if not 15 crit range and auto-confirms from the Kensai line.
If your GM is sadistic, this fight will be extremely uphill for you. The magus has a unique level of potential to become anti-anything by specializing himself. He could easily have the ability to detect you while invisible, some form of armor or magic that prevents sneak attack damage, etc. If your GM just tried to make a good character and didn't specifically target you it might be a little different. You could always ask for a special way to hamper him, via telling your GM that the fight seems rigged and you want to go study Magi weaknesses. Hire a caster with Legend Lore and read the target's past- the spell description indicates that any character over level 10 should provide some information.
Essentially, get native ninja with it- before Naruto, ninja where known for underhanded techniques that defied the moral high ground of Bushido. Show him the error of his ways. Constantly disarm him. Use Dirty Trick. Intimidate him with knowledge of his family and how easy targets they could be.