Never refresh hard nodes. It is a waste of crystals. It makes you feel like you have accomplished something faster but in reality you are usually still waiting on gear by the time the character is 7 star, which means you spend up the shard farm for nothing as the character isn't useable at that point anyway.
Use known drop rates of gear and shards to calculate how much time it will take you to get the character in question to your star/gear goal, and only spend crystals on gear/hard node refreshes if you absolutely need the character sooner than that calculation shows.
Spend crystals on energy refreshes. This game is a marathon unless you are willing to spend.
