Armor Breaker

Rogues dabbling in the ways of armor busting have shed the use of knives and rapiers in favour of heavier, more blunt weapons. Not exactly subtle, but sometimes you need to use heavier measures to make things work. Going for weak spots and smaller joints where the plating wasn't reinforced, they use weak points in armour to break it open wide and allow easy access to whats underneath.

Level 3: Swift Hammer

As part of this subclass, you learn to use bludgeoning weapons swiftly and effectively. Any weapon that does bludgeoning damage and doesn’t have the heavy property has the finesse property for you. 

Level 3: Armor Smash

When you land a successful sneak attack with a bludgeoning weapon against a target, you can sacrifice a number of sneak attack dice equal to your proficiency bonus to break its armor. For each die you sacrifice, you reduce the target's AC by 1. This condition can only be applied to one target at a time and lasts for one minute. If you choose to break the armor of a second target, the effect will end on the first. 

You have advantage against a target with it's armor busted

Level 9: Big Target, Big Weapon

Even when your attacks don't connect great, you're still able to deal lasting damage. Hard not to when you're using such a big weapon.

When you miss a target affected by armour smash, you can use your BA to deal half your sneak attack dice to the target.

Level 13: Forceful Shattering

When you break a creature's armor or hit a creature whose armor is busted, you can choose to knock it prone. While knocked prone this way, they can’t get up until the start of your next turn. This can be used once per short rest.

Level 17: Vulnerable Exterior

When breaking an enemy's armor, you can choose to hit in such a way that shrapnel flies to other enemies.

Roll half the number of sneak attack dice sacrificed when breaking the enemy's armor. Each enemy within five feet of the target takes piercing damage equal to the amount rolled. If the target is not wearing armor, i.e., has natural armor, you cannot roll this additional damage. Additionally, an armor-broken creature has -1d6 to all Strength, Constitution, and Dexterity saving throws.