You have specialized training in taking down massive threats.

Light-Footed

Beginning when you choose this archetype at 3rd level, when a large or larger enemy makes a melee attack against you, you can choose to spend your reaction to impose disadvantage on all that creatures attacks against you until the start of your next turn.

Slippery Target

Starting at 7th level, you can avoid the grasp of larger creatures. Whenever a large or larger creature attempts to grapple you, you can use your reaction to immediately make a melee weapon attack. If the attack hits, you can add half of the damage dealt as a bonus to your Strength (Athletics) or Dexterity (Acrobatics) check to resist the grapple.

You can move through the space of a large or larger creature without provoking opportunity attacks from any creature, but you must end your movement in an empty space.

In addition, you can add half your proficiency bonus (rounded up) to your Initiative checks.

Ride the Giant Down

Starting at 10th level, you can enter the space of larger creatures to grab onto them. Your Strength (Athletics) checks lose the restriction of targeting creatures no more than 1 size larger than you. Instead, any time you grapple a large or larger creature, you do not reduce their speed and instead climb onto them. While you grapple a creature in this way you gain the following benefits.

• Whenever the creature moves, you are pulled along with it, staying in the same part of its space.
• You can move freely within the creatures space.
• If you deal slashing or piercing damage with a weapon to the creature on your turn, you have advantage on all checks made to resist being grappled by the target and to maintain the grapple.
• The creature does not suffer the usual negative effects associated with being grappled (It's speed is not reduced).
• All melee attacks you make against the creature deal extra damage equal to your Strength modifier.
• Whenever you are targeted by an attack from a source other than the creature you are grappling, half the damage you take is transferred to the creature you have grappled as long as the attack roll would also hit the grappled creature. This effect does not occur if the attack beats your AC by 5 or more. If instead the attack misses your AC by 10 - your Dexterity modifier, the attacker must remake the attack roll with your grappled creature as the new target.
• Note: You cannot pin a creature using the Grappler feat if it is more than one size category larger than you.

The grappled creature does not need to use its action to break your grapple. It can instead attempt to grapple you with one of its attacks as normal. If it succeeds, your grapple on the creature breaks and you are instead grappled by it. Anything that forces you to move away from the creature more than 5ft will automatically break your grapple.

Monster Wrangler

Starting at 15th level, you have learned to take advantage of your position atop enormous monsters. On your turn while you are grappling a large or larger creature with the Ride the Giant Down feature, you can use your action to try and wrestle your charge into doing something you want it to do. The target must succeed on a Strength (Athletics) check opposed by your own Strength (Athletics) check. If you win the contest, you can pick one of the following options.

Direct the Swing
You force the creature to use an Attack action for free on any other creature in range. This can take advantage of any multi-attack or special natural weapon attacks in addition to normal weapon attacks so long as those attacks are not use-limited.

Bring it Down
You can force the creature to move up to half its movement speed in any direction, or you can knock the creature prone. If this forced movement would cause the creature to collide with another creature of at most 1 size category smaller or larger, the creature must make a Shove attack (no action required) against that creature to knock it prone or to push it back for the rest of the forced movement. If the creature fails this shove check, its forced movement immediately ends.

Go for the Eyes
When you succeed on your Strength (Athletics) opposed check, you can immediately make a single attack against the creature with advantage (no action). If the attack hits, the creature has disadvantage on all attack rolls, ability checks, and Dexterity saving throws until the end of your next turn. (Note: The creature need not have eyes necessarily for this option to be effective, as it can describe other physical means of disruption, or just being a painful nuisance on the back of some giant aether slug)

Adrenaline Junkie

At 18th level, the sheer thrill of battle keeps you energized and ready to fight. When you start your turn with less than half of your HP maximum while grappling a large or larger creature with Monster Wrangler, you regain hit points equal to your constitution modifier.
In addition, whenever you succeed on your Strength (Athletics) check to use your Steer the Beast feature, you immediately regain hit points equal to half your fighter level + your constitution modifier.