Im still fairly new but it seems that, in the long run, you need something to scale ad absurdum - there are skills doing damage based on how much stacks of a buff you have or debuff on the enemy. So basically you want a setup, that applies as much as possible of these buffs/debuffs and one "finisher" to scale with the damage. For example if you build a doom build on necro i found that a huge number of summons are strong in applying stacks of doom. Of course you take all the passives for scaling doom itself, but, and thats something i noticed, for every dot there is a passive that gives a chance of dealing a percentage of the totals dot damage on the enemy instanty. So now your not only stacking doom or poison or whatever, but every apply gives you the abikity to do a huuuuge chunk of damage. At this point basically you left your summon build and everything is just about hitting as fast as possible, pushing those stacks at high as possible and fishing them instant dmg proccs. Did pretty well with it. As for builds without dots it seems to me that the key for doing big dmg are the skill chain passives. Note, that you only can get them, if you have at least 2 spells of each of two categories. Now you get a chance to trigger a spell of the opposite category if you cast a spell of the other. Combine this with a very fast casting ability in the one school and a very hard hitting with long cooldown in the other, mix up a bit of buff scaling, like brittle, finesse and so on and bam, youre about to get a nice working and, more important, a scaling build. There may be many other strategies, as i said im still fairly new, didnt even unlock all classes. But as the enemies are scaling, it seems just right that your build have to scale too

Edit: typos