So, he's a 7/7 Insect for 5 and 2 green (converted mana cost 7) with Trample. That already is OK enough for a good uncommon. It's his secondary ability that makes him a mythic rare. Whenever he deals combat damage to a player, you make a token that's a copy of Giant Adephage. This means that you can end up with an army of Giant Adephage.
How can we abuse this ability? The most obvious interaction with Adephage when he was first released was an Enchantment called Infinite Reflection from Avacyn Restored. When attached to Giant Adephage, it would make all of your other non-token creatures into Giant Adephage. Although Reflection costs 5 and a Blue itself to play, this would be a situation in which your sudden army of Adephages could pummel someone to death.
At the time, Trostani, Selesnya's Voice was the most obvious Commander to utilize Giant Adephage. Trostani's Populate ability could make copies of the Adephage tokens. Mayael the Anima decks would use the Adephage as a target for her ability to put power 5 or higher creatures directly into play. But the best Commander for Giant Adephage turned out to be Xenagos, God of Revels, who would be released a short time later in Born of the Gods.
Xenagos has an ability that allows you to give a target creature haste and +X/+X where X is that creature's power until end of turn. Considering that you get to give the Adephage haste, you can do a lot of damage in a hurry. While it's hardly a staple in Xenagos decks, it is one card to consider for the deck.