In my opinion, what gives the best results for power is using training potions. But you should first try to collect all the ninja assist cards you can, which means you need to collect command flag frags to make command flags to recruit them. Don't level up any of your cards past assist rank level 5 unless you use that card on your teams regularly.

I would suggest getting and leveling up the Tenten card tho, since you can put her as an assist on any team and it will work. But you don't need to max out her assist rank to like level 9 or whatever the top level is. Just get to 6 or whichever level you need to max out speciality rank crit bonus to 20%.

Otherwise keep all your cards at lvl 5, it's diminishing returns to use more training potions to get them higher unless you already have every card unlocked. So, every time you get a new card, get it to level 5 and then start collecting potions and flag frags again for the next one.

Top priority for power:

- Training Potions

- Command Flags

Medium priority:

- Myuboku training scrolls (the 3 types)

- Refines and charms

- Rune stones

- Secret Scroll (scroll pages/mantra elixirs/plate fragments)

Low priority:

- Battle armor

- 8 Inner Gates