A counter to Mercy would be Spirit Fox. A couple potential counters for Maw are Gob-Chomper with empowered double damage to monsters, and Sunweaver with Monster slayer. 2x Gob-Chompers would do enough damage to take out Maw even with a life boost from Mercy; provided that it didn’t fill up and cast. Not sure how to make a team around that though. As for Sunweaver, here is an old team that does quite well:
Sunweaver
Naga Queen
Aziris
Wight
The idea is for Sunweaver to continuously cast on itself. Since its spell gives 6 mana, it only needs 7 mana to fill. By using the +2 blue banner, and 2x water link troops, it only needs 1 blue match to refill. With 3x blue troops for water spirit, and 3x undead troops for Undead captain, Sunweaver will boost its stats by around 16 every other turn.