To get there, theyre using a variety of mana accelerant cards like Seething Song or Mana Geyser, and pairing them with cost-reducers and ways to draw cards while comboing off
Similarly, old werewolves transformed back to their front side when a player cast two spells on any turn, while new werewolves transform back when it becomes day, which only happens if a player casts two spells on their own turn
if you want a general zombie commander, there are better options
If you want to push Adventures a bit further, specific Adventure decks also did well during Throne of Eldraine's reign over Standard
If you are instructed to put a card that isn't a double-faced card onto the battlefield transformed, it will not enter the battlefield at all
Lightning Greaves and/or Swiftfoot Boots can keep Meren itself safe, and the likes of Sol Ring, Kodama's Reach, Explosive Vegetation, Commander's Sphere and Birds of Paradise are all solid mana ramp options