There is a certain point that you can reach during the game’s regular story where you can use some tricks to infinitely level Steiner. This trick takes place early in Disc 2 in Alexandria Castle right after Steiner and Marcus escape their suspended prison cage.
In order to make this trick work you will either require a controller with a turbo feature or to utilize the scripting used for the Hail to the King Trophy.
Rock the cage back and forth using Left and Right on the D-Pad to escape. Equip Steiner with the 
Blood Sword which will cause all of his attacks to heal him for the same amount of damage that he does to the enemy. This will essentially give him infinite health when fighting low level enemies.
Position Steiner in the center of the background in a position where the Alexandrian Soldiers will continue to approach from the right side of the screen.
This will cause Steiner and Marcus to repeatedly be thrown in to battles. Marcus will eventually die but this is a good thing – it will allow Steiner to receive all of the experience points.
You can continue to battle as Steiner will continue to gain levels and experience. You can even walk away from the PlayStation 4 and let it keep going if you utilize a Turbo controller or the script mentioned above.
This will only level Steiner up but it can still make future battles for the remainder of the game that much easier. A brief video of this tactic is included below:
