"what should i focus on to get more power": Mt. Myoboku, Ninja Assist, Refinements, Charms... these gives amazing power... rest is... lets say less rewarding I guess.

"what's hashirama best breakthrough": depends on your team. Barrier if you use Konoha ninjas, Y if not... For passive I prefer immunity to debuffs over the immunity to ninjutsu and heal. And Y chase all the way... increased cooldown is way too good to leave it behind.

I still prefer immunity to debuffs, as Kushina suppresses immunity for 1 round, then Hashi is clean again. (If not hit by Edo Itachi while suppressed etc.)

Immune Hashi will also help you against ninjas like Rinne Sasuke. Its not nice when your Hashi with high chance to cause immobile on standard attack start hitting your team xD.