Sasaki was able to hold the Gate so well because Medea had added a bunch of stuff to defend her Temple, he can't actually handle Heracles at all
"Q. Lancer, Caster, Assassin, and True Assassins; all with normal attack and Noble Phantasms of B or lower. If they fought against Berserker, wouldn't it be a one-sided fight? Or are there any of them that can match up against him?A: Assassin and True Assassin by themselves wouldn't even be a fight. ...Also, in the game, Assassin was able to push Berserker back because of the terrain advantage and support from Caster (weighing down Berserker's body). ...Well, since the compatibility of Assassin and Berserker is horrible, the fact that Berserker was even pushed back is enough to say "whoah, Kojirou's amazing".
This is the assassin version from FSN, the one that isn't even actually Sasaki.