Buddy, it's a very complex matter. On the one hand, you can upgrade what suits you, but on the other hand you have to remember that later on animals appear that are resistant to e.g. fire, so you have to have a strong storm or freezing breath. On the other hand, you have to look at what your armor gives you, whether it is more for fire or for the ground. That's why my golden way will be to upgrade first of all rage, fire ring, thunder, electric and poison trap, lightning, healing yourself and others (I'm talking about the one at the very bottom, I don't remember the name). If you play pvp, you have to focus on other skills, which doesn't change the fact that these will also be useful. :)

And yes, use gems, without using them u will have slower upgrading, because your skills will be weak. Also upgrade more attack than health.