For me, she was not used until I had her fully traited. It’s the Empowered trait combined with her color flip ability that makes her top shelf. Otherwise, she is just a clenser that has to be powered up.
In my opinion, there’s a clear reason why spirit fox came out and it too had empowered… it’s for a player to have a response to a defence with Mercy full trait on it! Look at them both to appreciate the ying / yang.
Back to Mercy (full trait). She pairs very well with anything requiring a lot of yellow… and then you can also run the solid +2 yellow banner if you have a number of things requiring yellow. If you time her well with purple/yellow lined up together, she can insti-charge a Maw or Infernal King, or gorgotha, etc.
You can use Mercy to fuel Valkyrie, as valkyerie is gen’ing blue (and souls). Though you would put Valk in to charge another card first (like behemoth, or Queen Mab), mercy can catch the overflow mana… and valk has water link, so helps gening more there.
I scrolled through the present top 20 leaders and looked at their defense teams. Here’s some ideas per the top players in the game who are using Mercy (again, full trait teams).
Gorgotha
Behemoth
Valk
Mercy
Great Maw
Infernal King
Mercy
Alchemist
Great Maw
Infernal King
Sheggra
Mercy
Great Maw
Bone Dragon
Infernal King
Mercy
Great Maw
Bone Dragon
Famine
Mercy
Great Maw
Valk
Mercy
Queen Mab
Soothsayer
Valk
Queen Mab
Mercy
Zephyros
Marilith
Mercy
Dryad
Good luck,