It spawns one large Minion wearing a full suit of armor.
The Mega Minion is mostly usable as a defence card as it has high damage but is slow. However, it is often used on the offensive as a support card, being able to clear out small units and deal moderate damage to everything else.
Its high damage and moderate hitpoints allow it to be able to defend relatively well against many ground-based pushes. His defensive value is even greater since no spell can take him out for a positive or even neutral elixir trade, unlike Minion Horde and Minions. However, it can't deal with swarms very effectively due to its slow attack rate.
Mega Minion is a very strong counter against Balloon. His moderate hitpoints allow him to survive spells from Zap to Poison while his damage allows it to quickly deal with Balloon but it won't prevent any death damage.
The Mega Minion pairs well with the Lava Hound since it deals with any defending units trying to destroy the Lava Hound while the Lava Hound soaks most of the incoming damage. When the Lava Hound bursts, the Mega Minion's moderate health will allow it to tank the Crown Tower while the Lava Pups deal massive damage to the tower.
Naturally, the Mega Minion fits in very well with the Double Dragon strategy, as it's high damage allows it to quickly deal with major threats such as a Musketeer or Wizard, something the Baby Dragon and Inferno Dragon can't do.
To defend against a Minion Horde, the player can place this card in the middle of their side so the horde will be pulled to the centre, increasing the length of their way to the Crown Tower and distracting them and possibly to be in range of that second Crown Tower. The Mega Minion will take out 2 or 3 minions while your Crown Tower continually attacks. However, it is probable that you will still suffer some damage so it is best to use splash damage.
If ignored, the Mega Minion will be able to reach the Crown Tower and attack twice at tournament standards. However, this rarely happens since he can be distracted very easily. A Zap, Arrows, Fire Spirits, or an Ice Spirit will make the Mega Minion no more threatening than a kitten because the Princess Tower will kill it with the assistance of those cards.