I had a bug when entered the garden- the door didn't open and I had to bash it for 5 min.
Then everything went normal:

Four warriors party with crafted 2hand axes, scroll soulsap.

1st round- approaching the dragon and rage; the dragon stuns two of my warriors
2nd round- 1st warrior uses Nullify Resistances and soulsap scroll; 2nd warrior- Crushing Fist and Flurry- the dragon is dead. That's all was needed- one flurry.