An ability that triggers "whenever you fully unlock a Room" triggers when a door becomes unlocked and the other door of that Room is already unlocked, or when both doors of that Room become unlocked simultaneously
On the high end, Ezuri could just spam Sprout Swarm and create a 10/10 Blighted Agent, and Meren just needs an Altar of Dementia and Avenger of Zendikar to acquire enough experience counters to reanimate any creature going forward
Need proof that this strategy works
Anytime you attack with one or more Goblin creatures, you get an extra 1/1 Goblin creature token tapped and attacking
Every time something enters from the graveyard, Kotis grows larger with +1/+1 counters, which means pairing it with recursive creatures like Gravecrawler quickly turns this Champion into a massive threat that cant be ignored
In this case, everything other than Ad Nauseam and Griselbrand, the commander, is 33, so when you draw your deck, you will lose around 30 life