Mamoswine seems like a bit of a typical stage 2 to be honest. Its first attack, 40 for three (two with a DCE), does 40, and you can flip a coin, if heads, the defending gets paralyzed and you discard an energy on the defending. Its second attack, 60 for four (three with a DCE), can give you the option of doing 100 damage and 40 to one of your opponent's bench. If you do, Mamoswine must do 40 damage to itself. Yep, right now, this card looks mediocre at best.
140 HP is impressive for a stage 2. Metal weakness is a considerable pain, the lightning resistance is fantastic, and the retreat cost is just way too heavy.
Overall 3/10 in modified, 6/10 in limited, and 2/10 in unlimited
In modified, this card is not really a satisfying card to use right now. It relies a lot on DCE, so it is pretty slow to attack with, and that is pretty much the big reason. Many SP decks, and other fast decks, can stop Mamoswine before it fully sets up. The retreat cost is also a huge pain, so it is going to be tough to pull back when it gets heavily damaged. Its weakness to Dialga G is also painful, especially since it is plus 40. Yet, despite these downsides, this card has its pluses. For one, it is tough to take out with that high HP. It also has lightning resistance, so Luxray isn't going to KO it easily. It can even do some impressive damage, 100 at the most to be exact.
In limited, this card is a bit better. Once again, high HP, lightning resistance, and high damage really makes this card an impressive one to use, but speed is the big issue once again. With no DCE here, it won't be that easy to attack with. Therefore, one should be weary when using it.
In unlimited, this card is back to being below average. Speed is once again a big issue, and so is the weakness and retreat cost. Many cards can easily counter this card as well. Still, despite this, Mamoswine is not entirely useless. It still has the potential to hit for high damage, and the resistance and HP is certainly something impressive to look at.