Path of the Giant Slayer

This Barbarian is one who is not intimidated by a gigantic creature, and in fact is ready and willing to slaughter them. They live to fight creatures larger than themselves, by big margins. These barbarians are often a kingdom's only hope against a rampaging ancient Red Dragon, an invasion from a band of Hill Giants, or the one and only Tarrasque.

Stopping power

At 3rd level, a giant Slayer barbarian has learned the basics of how to deal with a size disadvantage. You can grapple any creature, regardless of size, and if that creature is 2 or more sizes larger than you, they have disadvantage on any attempt to resist or escape the grapple.

Matching power

At 6th level, you have subverted the reach and speed advantage of a large creature. Any melee weapon attack you make has an additional 5 foot reach, and you gain +10 feet of movement

Gargantuan manuvering

At 10th level, you are able to use the creatures large size against it. You can pass through the space of a creature that is 1 or more sizes larger than you, and if the creature is 2 or more sizes larger than you, you can make a grapple check as an attack option to attempt to climb it. If successful, you move into the creature's space, the creature has disadvantage on attacking you, you have advantage on attacking the creature, and any hit you land on the creature is treated as a critical hit. 

As an action, the creature of which you are climbing can use it's action to throw you off with a successful Strength (Athletics) or Dexterity (Acrobatics) check against your Strength (Athletics) check. If the creature succeeds, it tosses you off, and you take 2d6 bludgeoning damage and fall prone and 5 feet away of the creature.

Golaith conquering

At 14th level, you are able to best creatures larger the buildings consistently. Whenever you get attacked by a large or larger creature, you can use your reaction to make an attack against that creature. Aditionally, whenever you make a melee weapon attack against a creature, once per turn, you can force it to make a dexterity saving throw with a DC equal to 8 + your proficiency bonus + your strength modifier, and if it fails it falls prone. A creature 2 or more sizes larger than you has disadvantage on the saving throw.