I just found the critical approach of playing the Buff Knight Endless Mode - Juggling the enemies.
If you read the hints of the Buff Knight game play at the beginning, there is one important way of striking the enemies apart from the normal melee attacks OR magic attacks - Juggling.
To perform the juggling, you have to keep tapping the enemies when they are in the air. That's the tricky part of the Buff Knight game. Enemies coming from the ground. They might be sent flying if struck by the magic lightning OR melee attack. When that happens, make sure your finger is unstoppably hitting the flying enemies! If you have your melee attack upgraded, most of the time the enemies would be killed before landing!
That's pretty much the main idea of the Buff Knight. Even the big boss is not spared off!
The picture shows my achievement in the Buff Knight Endless Mode - reaching 40,000 meters++ with tons of potions. Basically it can really go endless if your fingers are not tired ;)