Monster of the Deep is exactly what it sounds like. It has a long way to go before it makes it to the battlefield, but when it reaches the field nothing is bigger. You must invest 6 energy (2 blue and 4 colorless) to play and all you get a 3AP/3HP ally for your troubles...unless you bench it! Like Millimo and Sea Monster, this card has the ability Develop, but unlike them it gains 2AP/2HP each turn it is on the bench. After three turns it will be 9AP/9HP, larger than any other ally. The problem with this card is that you have to wait until you can pay 6 energy to bench it, at least 6 turns unless you play an energy-increasing card like Energetci Wizard. Develop is a fine ability for the first few turns, but unless you can truly control your opponent, it's hard to justify waiting so long to get an ally. Still, two undefended hits with it will be enough to win a game, so you could probably find a way to work it into a deck.
Strong against: opponents that are slow-developing or have no bench hate.
Weak against: aggro and bench hate cards like Apocalypse.