To raise attack:
• Use a Pokemon with Synchronize with one of these natures:
- Lonely (+Atk, -Def)
- Adamant (+Atk, -SpAtk)
- Naughty (+Atk, -SpDef)
- Brave (+Atk, -Spd)
Max out the IVs using hyper training in Hau'oli City and Max out attack EVs in Poke Pelago. For remaining EVs, since you don't want defense EVs, Raise its HP, since it can't be "raised" by Beast Boost, and Speed, since it's max speed stat anyways is 137, with a supporting nature.
To make Atk>Def:
Same as above, try sticking to Lonely nature.
Also, You might want one of these other defense-lowering natures:
- Mild (+SpAtk, -Def)
- Gentle (+SpDef, -Def)
- Hasty (+Spd, -Def)
That's all I can recommend.
Also, the chances of this happening is EXTREMELY slim, as Stakataka's Max Attack stat is 397, and its minimum Defense stat is 384.