1) It may seem a little silly, but I always make sure that my spirit animal bond is at its total % before I do anything, I could be totally wrong but I think it helps your animal become a better fighter (as well as give you rewards).
2) Go on your level quests they will de the ones in the bottom right corner. Those quests will give you great beast amulets; like the panda for extra health boosts.
3) When you go to the armory hover your mouse over the objects in your inventory on the right so that it is easier to compare them with the ones you have on your hero.
4) For each item that you have in your inventory will have an arrow in the top right corner that will be green and pointing up if its better that what you have on or red and pointing down if it is weaker that what you have on or it will be a yellow equal sign indicating that it is the same as what you have on. These arrow will change basted on the item's level, attack damage, or attack spod.
5) Sometimes when you try to sell something a unity system notification comes up and then you have to reload the page to play again. This usually happens if your inventory is full, so try to always keep it at around 20-30 items instead of 50, which seems to work. However if this happens there are two things that I know of that help: 1. if you still have room in your inventory buy something from Mustela's shop after that you're then able to sell items from your inventory. 2. If your inventory is full the best advise I got is to get the Spirit Animals app and sell your items there. The app is free just as a heads up. :)
6) If you don't what to spend money on a solo quest and aren't able to do the next level quest, then do a group quest. Battling as a group is lots of fun. You to get you lots of items as well as experience points. The more experience points you get, the faster you level up!
That is all the tips I can think of, I hope they're helpful.
Good luck to you all! I hope you have as much fun as I do in the world of Spirit Animals!