Obviously you can play it early as an above-curve threat that can trade up and start triggering whatever Zombie synergies you have, but the real power of Tainted Adversary comes in its second ability
Later on during the events of Godzilla Island though, Gigan would ally with Godzilla in fighting off invading monsters
Like the Jadelight Ranger above, it either fills your hand, smoothes your draws, or becomes an increasingly significant threat
Training Grounds just speeds up that process and might remove one of the elements of the chain, making it easier
However, one suggested option from YouTuber PubStomp MTG is Rise of the Dark Realms, which can upgrade your deck fairly and win the game for you the turn it is played
Good as it is, once this cards on the field, you only need to pay mana whenever you want to start drawing cards