Damage Problem?

I am a level 99 mage and I have always had a problem with not dealing enough damage like when me and my friends fought the boss in Reincarnation and recently bosses in the Legendary Island. I never understood what the problem was. How was I not doing enough damage? Even a friend a couple levels lower than me was doing a lot of damage but I do not really get the concept of it. What is considered even a good build? My friends say I should look for builds on the shop but I am not sure what I am looking for. I am well on spell damage, mana regen, medium on speed, have elemental attacks. But what is the problem? How come I apparently cannot deal enough damage? What is the key essential that helps on dealing damage? Do I need a mythic build? What should I focus on? I really don't have a clue on this. I'm not yet too experienced with builds and such. Please let me know what I am missing and probably doing wrong. Thank you.

 

Bob takes little elemental damage (for the most part)
And depending on your build, weapon choice, and the boss's defenses, you won't be able to deal the 15% damage needed for a drop

 

whats your build

 

Bob's Reincarnation has "rainbow" defenses so it is advised to bring a powerful neutral damage weapon.

 

What about high health bosses like Legendary Island bosses, with really high health bosses what should I be focusing on more?
I only have trouble on higher healthed bosses and I'm not sure wht I am doing wrong. If elemental attacks are the problem do I need a weapon with a good amount of neutral damage?

 

Use weapons with elements that the boss is weak to but do not use elements that the boss resists, if the boss resists all elements, like Qira and the Virus Doctor, use a neutral damage weapon.

 

Oh, no wonder I did like zero damage when I fought the Virus Doctor earlier. Thanks a lot!