Spend Theonite first for Relics and then energy refreshes when you really need it. Always buy the 10+1 relic because you get 1 bonus relic with extra rare reward rate.
Save all fighters you get, they can be used for evolving fodder later. Currently sacrificing them for experience is not worth it. Always level the fighters up before using them as fodder as they give skill points when levelling up.
Skill points should be prioritized into the ground combo extension, and depending on their SA, the skill tree or the move tree.
Strong Bronze fighters include: Cold Stones, Bad Hair Day, Rusty, Beat Box, Headstrong
Strong Silver fighters include: Doublicious, Inkling, Peashooter, Bad Ms Frosty, No Egrets, Oh Mai, Blood Drive, Scarlet Viper
Generally a good idea to invest into a bronze (cold stones highly recommended) as they take less skill points and exp to level. Gold growth is significantly slower and costs a lot more to upgrade skill tree.