Here's my way to get an S rank in about 100 seconds:

Free Battle Map:
Unguis Lapis A(the very first one)

Player deck:
Kranz

Cards in this order:
Savitri
Partisan
NA Guard
SW Attack
Beat

Then proceed to fill up the deck.

Dummy deck:
Endu

Cards in this order:
Rag Rappy
Rag Rappy

Proceed to fill up the deck.

Rule settings:
Min and max dice = 6
Shuffle unactivated.

If Endu starts first, skip his turn.

On Kranz's turn, equip the Savitri, and move forward one step.

On Endu's turn, summon a rag rappy in front of Kranz, and another rag rappy to the left of the first one. Use 3 points to move Kranz to the right of the first rag rappy. Use the final point to attack Kranz with the first rag rappy.

Make Kranz defend with NA Guard.

On Kranz's 2nd turn, equip the partisan, and slash away with a SW Attack + Beat combo and all 3 opponents.

I hope this is helpful.