There are lots of events that can help you, both spending and non-spending events. Like what was said above, Lucky Dice or Lucky Board is a very good event to spend on, especially when the secondary prizes that you can redeem are things that you want (whether they be ninjas or medium-level refine runes for power).

What will help with power in a cost effective way is to spend during events for items such as refinement runes, Charm Materials Pack, and Cave Key Gift Pack.

Also being able to stack events and get more than just spending for what you want is always good. For example, there are events where just for spending ingots, you can get rewards. There are also events where the action of recharging can award you redeemable prizes.

Always look for stackable opportunities to get the items, then wait for events like refinement rebate to use them.

I know it is a lot to take in, but I hope it is helpful. If you need me to clarify, just let me know.

~Jib