Would appreciate some tips on how I can make my build better

Okay, so I’ve been playing this game practically since it’s inception. But I’ve never really understood how the crystals worked, or the mythics for that matter. But, I finally google searched the game and saw that there was a wiki for the game. I made a blight build, after hours of messing around with the crystals, constantly clearing the games cache, and redownloading my profile. I finally managed to get blight/elemcrit on every peice of gear… But I’m still confused, do certain weapons play a factor? Is there anyway I can improve my build? I’ve seen videos where people do 1-2 billion damage, and I just assumed they must have been hacking. But I’ve done as much as 42million from the poison aoe from blight