How core guardian should be nerfed without nerfing DH or Firebrand:
Righteous Instincts now grants 30% critical chance increase, lowered from 50%.
This now makes it so, with Valkyrie and with Retaliation and Fury, you have a 55% critical hit chance, lowering the number of critical hits you make by 20%, thus decreasing overall damage during bursts by roughly 25%. If guardians on mass swap to marauder instead of Valkyrie, they will have -6.32% power damage, and -22.67% critical damage. This will make it so that while you are bursting, with fury and retaliation, you do ~18% less damage than before. This does not affect the meta DH build. This does not affect the meta Menders Firebrand build.
How Deadeye should be nerfed without hurting Core Thief or Daredevil:
Silent Scope no longer grants stealth while rolling with Rifle. Shadow Meld no longer removes revealed. Deadeye's mark now needs to hit the target to trigger any on-hit traits.
No explanation needed.
How Holosmith should be nerfed without hurting Scrapper or Core Engineer:
Corona burst's damage is lowered by 20%. Holographic Shockwave's radius is lowered from 600 to 450. This no longer acts as a blast finisher. The knockdown duration from this skill is decreased by 50%.
How Elementalist and Mesmer should be changed act quite differently because the issue is the core specializations. Multiple core mesmer traits are very strong but multiple core Elementalist traits are very weak. The changes must happen to these to sufficiently change problematic builds (or in Elementalist's case, a lack of any strong builds whatsoever).