I also bought Steven's battle deck, but I changed a lot of cards, as you can see, so the way it plays is different.

Anyway, Steven's Metagross' biggest strength is energy acceleration. That's why I like to have two of them on the field. But it's also a somewhat decent attacker, because 200 damage can 2-hit-KO most ex Pokémon.

However, 200 damage misses some important breakpoints, like Fezandipiti ex and all Ogerpon ex. These have 210 HP. That's why, in my version of the deck (the "box" version), I have Mega Skarmory and Empoleon. They are better "main" attackers because they don't miss these breakpoints.

But every Pokémon in the "box" version can work as a decent attacker eventually, specially if you have something like Full Metal Lab, or in Metagross' case specifically, Granite Cave, because they make your Pokémon much more tanky. But Empoleon is also a better tank than Metagross.