Balan, Wandering Knight is your best single creature for dealing massive damage, Kemba, Kha Regent can quickly build an army all on her own, and Raksha Golden Cub effortlessly turns your army into a game-ender
It weakens your opponents' boards, can do some serious damage to low-toughness tokens, and plays nicely with the Connive mechanic that runs throughout the Villain archetypes
That said, Hearth Elemental or more accurately the adventure on Hearth Elemental, Stoke Genius does quite a good Breakthrough impression
For Rin and Seri, I avoided tutoring (even Fetch Lands), selected almost exclusively cards that were on-flavor (cats and dogs tribal) or on-strategy (related to tribal), and prioritized playing as many cats and dogs as possible