Foriysian Totem

{T}: Add {R}.
{4}{R}: Foriysian Totem becomes a 4/4 red Giant artifact creature with trample until end of turn.
As long as Foriysian Totem is a creature, it can block an additional creature each combat.

Ancient Amphitheater

As Ancient Amphitheater enters, you may reveal a Giant card from your hand. If you don't, Ancient Amphitheater enters tapped.
{T}: Add {R} or {W}.

Pyroclasm

Pyroclasm deals 2 damage to each creature.

Mountain

Stomp

Damage can't be prevented this turn. Stomp deals 2 damage to any target.

Bonecrusher Giant

Whenever Bonecrusher Giant becomes the target of a spell, Bonecrusher Giant deals 2 damage to that spell's controller.

Bonecrusher Giant // Stomp

Prismatic Lens

{T}: Add {C}.
{1}, {T}: Add one mana of any color.

Stinkdrinker Daredevil

Giant spells you cast cost {2} less to cast.

Magma Jet

Magma Jet deals 2 damage to any target. Scry 2.

Thundercloud Shaman

When Thundercloud Shaman enters, it deals damage equal to the number of Giants you control to each non-Giant creature.

Awaken the Ancient

Enchant Mountain
Enchanted Mountain is a 7/7 red Giant creature with haste. It's still a land.

Cast Off

Destroy all non-Giant creatures. (Then exile this card. You may cast the creature later from exile.)

Realm-Cloaked Giant

Realm-Cloaked Giant // Cast Off

Flame Slash

Flame Slash deals 4 damage to target creature.

Sunrise Sovereign

Other Giant creatures you control get +2/+2 and have trample.

Borderland Behemoth

Trample
Borderland Behemoth gets +4/+4 for each other Giant you control.

Bloodfray Giant

Trample
Unleash (You may have this creature enter with a +1/+1 counter on it. It can't block as long as it has a +1/+1 counter on it.)

Giant Harbinger

When Giant Harbinger enters, you may search your library for a Giant card, reveal it, then shuffle and put that card on top.