Tough card to find use for. Not useless, but there are just better option. An example would be his brother Avalanche Giant, which i would say is better in every way, shape and form. Costs 1 less mana, has more base power, and the same effect. But on top of that, if he gets blocked he breaks a shield. Since giants don't have any direct support, i would never use Nocturnal Giant over Avalanche Giant, despite having more power when attacking. Since it can't attack creatures, the power attacker will only ever be relevant if Nocturnal Giant gets blocked by a massive blocker, and you dont really see those very often (pretty sure Valkyer Starstorm Elemental is the only one of relevance). So it is better to just have a higher base power. But crappy stuff aside, if we were to use Nocturnal Giant in a deck, just for the heck of it, i think i know how!
Not being able to attack creatures, Nocturnal Giant is there pretty much only to tear away at your opponents shields, and to remove their defensive lines with its massive attack. So i would build a heavily defensive deck, with a lot of blockers. This would protect you against rush and aggro, and would slow down the match so that you can get your mana going without dying. 7 Mana and no speed attacker makes Nocturnal Giant very slow, so Soulswap is absolutely needed, in order to get it out as fast and cheap as possible.
We are also gonna need another civilization to help us out with blockers. Obviously light comes to mind, as they have the best blockers, and with cards like Sanfist and Skysword. But i would rather look towards water, which can not only provide you with blockers, but also with draw power and removal. Being able to Soulswap Surfers and Coriles is also no joke. So the deck would have the gameplan of being defensive until reaching 5-6 mana, and will then start taking control of the match, and eventually summoning Nocturnal Giant as the finisher. Here is the decklist i came up with:
x4 Nocturnal Giant
x4 Faerie Life
x4 Bronze Arm Tribe
x4 Aqua Guard
x4 Madrillion Fish
x4 Aqua Hulcus / Energy Stream (Just couldn't fit both)
x4 Phantom Fish
x4 Aqua Surfer
So there it is. Only 10 different cards, with 4 copies of each. Im pretty sure this deck would work just fine, atleast against non top tier decks. I guess depending on your meta, you could go more mid-range and remove the 4 Aqua Guards for a bigger blocker like Revolver Fish or Zeppelin Crawler, to be better suited against slower decks, though Aqua Guard is good bait for Soulswap. You could also tech in 1 of either Marine Scramble and/or Trash Crawler. Scramble works as a finisher against defensive decks, while Trash helps you make the deck more consistent, and also works great with Soulswap.