A dragon farmer is on my todo list, unfortunately the list keeps getting longer and I'm not crossing anything off :) 

Please show off your ninja, here or we can start a separate thread.  I have an IG20 and spider20 farmer only.

For Zavia build, my sources say she scales only from ATK, not Crit Damage. Like a bomb champion, except she needs CR. I built mine like so

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with ATK% gloves and ATK amulet. She did well in the Arena at the time, before she was replaced. Never tried to build her with CD, so i don't know for sure if it helps.

As for speeds, i'd like to build upon what harleQuinn already did:

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All Speed(2) rolls with mostly +2 and +3 gliphs. With 6* common SPD boots that's 290 speed. And try to eliminate 3* and 4* gear, as soon as you start farming 5 and 6  star gear, begin selling anything below that automatically.

With speeds around 300, you might have a shot to get to gold. But most importantly: always speed tune your team!

Your sources told you something wrong. Zavia's normal hit damage just works, well, normal. Her A3 skill Deadly Catalyst has 4.1 x atk as multiplier, and is calculated as any other hit. So you definetly need c.damage if Zavia is supposed to hit harder than a wet noodle.

On top of her normal hit damage comes the poison explosion - if there is any poison. That scales from the poison damage only and is not affected by c.rate, c.damage or attack. Building Zavia with 100% c.rate without c.damage doesn't make any sense.