Hamletback Goliath

Whenever another creature enters, you may put X +1/+1 counters on Hamletback Goliath, where X is that creature's power.

Plains

Obsidian Battle-Axe

Equipped creature gets +2/+1 and has haste.
Whenever a Warrior creature enters, you may attach Obsidian Battle-Axe to it.
Equip {3}

Mountain

Hammerfist Giant

{T}: Hammerfist Giant deals 4 damage to each creature without flying and each player.

Chromatic Lantern

Lands you control have "{T}: Add one mana of any color."
{T}: Add one mana of any color.

Brion Stoutarm

Lifelink
{R}, {T}, Sacrifice another creature: Brion Stoutarm deals damage equal to the sacrificed creature's power to target player or planeswalker.

Crush Underfoot

Choose a Giant creature you control. It deals damage equal to its power to target creature.

Blind-Spot Giant

Blind-Spot Giant can't attack or block unless you control another Giant.

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}.

Brighthearth Banneret

Elemental spells and Warrior spells you cast cost {1} less to cast.
Reinforce 1—{1}{R} ({1}{R}, Discard this card: Put a +1/+1 counter on target creature.)

Borderland Behemoth

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

Seething Song

Stinkdrinker Daredevil

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

Sunrise Sovereign

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