Cave keys, refines, ninja tools, threads, charms, costumes, mangatamas, purifications, ti tools, ninjas at home, summon cultimavtion, summon runes, myoboku, etc . Whether you are at 10, 40 or 150k power pretty much only thing that changes is basic -> medium -> advanced in front of refine/thread. You gain power the same way as you did before, the same way as everybody does at any level. Just do your stuff, hoard items, spend them during rebates.