Comments from the Finger: This is heavily inspired by the Blade Bravo of 3.5, and a certain build called 'I Might be Tiny, but You're Dead: The Other Killer Gnome.' I swear, we're never going to run out of inspiration from 3.5.
Prerequisite: Small size
Though titan slayers often adopt larger-than-life names, and perform immense feats, they are universally little folk, such as halflings and gnomes, which excel at battling foes larger than themselves.
As a titan slayer, you have learned a number of techniques to dumbfound larger creatures, so that you can target their vital areas and bring them down swiftly. You use your foe's size against them, weaving between their limbs, and striking when they least expect it. No challenge, and no foe, is too large for you, for as they say, 'The bigger they are, the harder they fall.'
Starting at 3rd level, while you are adjacent to a creature that is two or more size categories larger than you, you can use your bonus action to enter that creature's space. You remain in this creature's space until it moves or until you use your bonus action to move to an unoccupied space adjacent to the creature.
While you are in a hostile creature's space, you can use Sneak Attack against that creature, even if you do not have advantage against it or if no enemy of the target is within 5 feet of it. Additionally, while in this space, you have half cover if the creature is two size categories larger than you, and three-quarters cover if the creature is three or more size categories larger than you.
At 3rd level, when you make a Sneak Attack against a creature larger than you, you can forego half your Sneak Attack dice (rounded down, to a minimum of 1) to knock that creature prone.
Confound the Big Folk
By 9th level, you are an expert in the art of baffling the big folk, who are slow and unobservant by your standards. You have advantage on Dexterity (Stealth) checks made to hide from and sneak around creatures larger than you. You can also add half your proficiency bonus to any ability check that doesn't already use your proficiency bonus which you use to avoid, climb on, or confuse a creature larger than you.
Fear No Monster
By 13th level, you are fearless in the face of impossible odds. You can't be frightened, and, when you roll initiative, you have advantage on ability checks saving throws you make until the end of your next turn.
Fell the Giant
At 17th level, your weapon attacks against creatures larger than you score a critical hit on a roll of 19 or 20. Your critical hits also knock the target creature prone.