I just wanted to post about my build idea and tested since I think its really cool.

I've tested Hydrosphere with spell echo and can confirm that the final repeat does have a 100% crit chance and when using it with greater spell echo it also inherits the extra aoe and more damage multiplier.

Scion Archmage: https://pastebin.com/vFcRu4Zf

This is a (not that good) pob for an archmage scion that I used the interaction with. I stacked cast speed to make the build be able to clear maps a bit more smoothly but its strength is definitely in bossing. I defensive layers are pretty lacking and I have no idea how to improve them. Feedback would be much appreciated

Elementalist Archmage: https://pastebin.com/miFgCffP

This version has extra mana regen from the ascendency so you can use Agnostic comfortably.

In both pobs the custom modifiers are there to guarantee crit since plume of pursuit isn't working, divergent spell echo damage isn't applied properly, and greater spell echo more damage also isn't applied properly.

If anyone can make better builds please share, I'm too stupid to think about anything but archmage.

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Extra: There's a weird interaction when mobs run into the hydrosphere when it's already up. The first three pluses don't crit and the ones after will. No idea what causes this except for some weird spell echo interaction.