So, I haven't played DnD and was pretty confused at first with how everything was supposed to be, but after playing for a while I understand a bit, here's what I got so far in case anyone else is also confused, feel free to add more advices if you have any.

In this game you use mana to cast skills and attack, there's one golden mana that is used for the party skill. The icons that the characters have with the purple edges are the passives of each character, they don't use any mana, the ones with white edges are actives/skills you can use with mana, and the ones with golden edges are the party skills, you can only use one per encounter and per character, meaning that you can't use two or more when fighting an enemy. 

There are 3 colors for the dice, red for damage, green for healing and blue for shielding, each have three types of dice, one with 4 sides, one with 8 and another with 12, when you see a spell saying "Deals 5D4 Damage" it means that the game will roll a 4-side die 5 times (5D4 means 5 times a 4-side die) and add the results together, so you'll be wanting to look for something that rolls high numbers on both numbers and pair it with good passives.

After some encounters (enemies) you'll be given bonus items, from more mana to add a number when you roll a die and more, so if you're choosing, for example a +4 to a red D4, please make sure that the skill you're using uses the same die, also, I don't know if it's a bug or not, but I've faced two final boss in a row, so be careful with that.

This game is just trial and error since we can only see the abilities each character have while playing them, so you'll have to play a good amount if you want to win.