What builds are still viable and what level should I have my first build?

I'm level 24 and I know max is 30. I haven't completed crucible, or got to crucible 5 to get the respec statue.

When I look up people running crucible or just defeating the night and day temple boss, they all have builds that basically make them invincible or have unlimited focus, etc.

Personally I'm trying to go for a mage build. I like the wands and staffs in this game and think cronus is a cool sword as well. However the only build I found fitting this is stacking focus regen on every piece of equipment I have and I only ever found 1 opal. Can anyone else suggest good mage builds or ways I can build my character, as well as what level I should be obtaining these builds, or when exactly I should have a build?