The Summoner Alric.

He is one of the hottest summoner in Bronze | Silver league, since few monsters can counter magic. He provides +1 magic power for mages. He is the most important card to rent/buy.

The Trio Trouble.

In my water deck this 3 cards is a must rent for me, specially I am grinding in Silver 1, competitively. You'll get. +3 speed with this Trio and 7 mage damage, and they really attack fast. You may also just rent for Alric (if you are just grinding to earn DEC.)

The Tanks

I usually use Dijnn as my primary tank. Since it has Phase and Void, In that way I can counter mage user like me.
If the mana cap is high, I usually put The Kraken as my first position and Djinn as my 2nd tank. Lastly my favorite card, Kelp Initiat, my little league tank. With just 2 mana cost, can win you a lot of battles.

The Supports and little leagues.

Here's the cards that I usually rent for my grind. Coral Wraith for Sneak and kill back lines. Mermaid Healer, my primary backline support. Provide Triage, Medusa. My Stunner it is really good specially if other enemy has tank heal. Elvin Mystic. To counter magic user like me, since it has silence in level 4. Ice Pixe, my little badass sneak protector, with Spirit miner next to her. I did not include Venari Wavesmith, since I already have mermaid healer.