I didn't say the sorceress helped him block the attack. Geralt did block the attack but he was pushed back by the monster. Radeyah had to interfere to give geralt the time to drink a potion.
And even if the monster weighs 60 tons, geralt isn't benchpressin him, the monster is pushing geralt.
Silver is effective against monster and not effective against humans, in the game you do little damage to humans with a silver sword, and you also do little damage to monster with a regular sword. The silver sword isn't as sharp as a steel sword, but he cuts monsters more effectively.
Silver is also less durable than steel.