If you upgrade the pet dragon to level 31, the damage of the pet will get doubled (fireball) it's primary attack skill. After that, every increase in upgrade will increase the damage again gradually like you're doing to reach level 30 with the requirements also increases.
You must decide how to use your power stone carefully as it is so hard to get. If you are using it in a unique sword, be sure it is the right sword or the sword you want most because upgrades that requires power stones cannot be transferred. It will become a waste if you're unsure so a good advise is to think about it more.
I can't decide myself cause I use all of my unique sets for now π
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