When you face a opponent with the same element as yours you have a certain resistance to each others attack. The element set converts your spells to a random element attack with all bonuses converted. Have an element affix pet rather than gear because you choose when the bonus becomes active. Before fighting check the element your opponent uses to see if it’s the same as yours. Commence round one but don’t press the pet icon until you figure the attack style. Once you are sure of how you would fight back then activate your pets bonus and hope your opponent is weak against that element. Check the damage your doing but do not press pet bonus untill you have to. Summoners have a little more freedom
But if your enemy has insane resists and a certain mythic that balances all resists ( including your weakest ). Good luck . But I would suggest crafting a legend " element" pet with good affixes that are helpful but you can survive without and put them on your items. I have a great eternal pet but now the attraction is wearing thin. You become reliant on your pet and not your build.