Blowing it up if its a source from a permanent (like Aura Shards!) just accelerates Green Slimes value
While a 3/2 for four mana isnt great, the ability to counter a wide range of effects (spells, activated abilities and triggered abilities) or destroy a creature or planeswalker when Ertai enters makes this card an amazing Swiss Army knife that can answer a number of kill-on-sight threats, often in a surprising way, as the fact that this card counters triggered abilities can really stymy a foes game plan in unexpected ways
You lose out on some good options by not having access to green, but there are some great white and Azorius merfolk youll have access to
In a deck that plays a lot of high-mana cards you get a good amount of value per card in hand for this, and it works especially well with the 4-power draw effects as you can churn through the deck
That said, most playgroups are perfectly fine with using proxies
The Black Gate The Black Gate is a nice utility land that can help you to attack a player freely if they have the most life