I think a berserk mechanic that builds up while the adds are stunned would be really cool. Similar to the ogres boss mana lanterns mechanics. So while the adds are under the effects of frost cage or dazing trap or petrify, they'd be building strength, and then when they come off of these effects they'd start losing strength. However if they kept under the effects of the spells for a total of 40 seconds they'd get stun breaks and start going totally berserk, essentially instantly causing a wipe, similar to the the mana lanterns again.
CC'ing the second add would also make Zharo more powerful, so the players would have to make a decision in how long they really want to keep the second add disabled for. I'd also not be opposed to starfall being stronger or even a triggered instant death. When we get more health from the next gear set this spell will become trivial to dodge IMO
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For this to work Zharo does need more health though. He should honestly be pretty much impossible with full foothills gear IMO. Forcing us to build up some gear from other bosses first would be good. Unless you're planning on adding a fourth boss, but I thought I read somewhere we'll only be getting three.