There's only two scythes of delay 502 at that level, was that a typo? Did you mean 501? If it is 502, then that may be the reason it failed, because the only scythes with that delay are level 40 and 53, and both outdated.

Personally I'd go for a Vendetta. They're really cheap these days and have highest damage for their level, which is what you'd want for this, since 2x Guillotine may quite possibly be enough to kill him if done correctly, although it would be harder at level 70. Still, it should put him down a serious notch. What Khajit said too, use blink/stoneskin before the fight, even if it just allows you a few extra hits, it may be enough, especially if those lead to another Guillotine.

However, if somehow you can get access to a Mercurial Kris for this (or Kraken Club of course), it might be worth considering that strategy. Still, 2x Guillotine should be performed with the highest damage you have on you, even after that you can switch to lower delay weapons.