Rogue Legacy 2

TLDR at the bottom if you aren't interested in mechanics.

The equipment system works like this:

Finding blueprints in chests in the castle allows you to buy that item from the blacksmith at the docks with Gold and Ore. There are 11 armor sets, each composed of 5 pieces. Equipping these pieces will, in addition to stat boosts, give you points towards a unity bonus. The unity bonus grants you set-specific bonuses at 3 thresholds: 20, 50, and 100. Every armor piece gives you 5 unity points except the accessory (trinket, sigil, band, etc) which grants double that. Therefore, wearing every piece of leather armor will give you 5+5+5+5+10 = 30 unity points, enough to reach the first unity bonus but not the second (greater than 20 but less than 50)

Once you've beaten the first boss, you'll receive a new currency and a new shop in which to spend it will unlock. The very first item on the shop list is Embroidered Investments. Buying a rank means that you will be able to find blueprints for improved versions of the armor sets, which have better stats, weigh more, and critically confer greater unity bonuses. (A +1 set for example will give you 10+10+10+10+20 = 60 unity, enough for that second bonus, or 3 level 1 bonuses if you mix and match!)

You will need to wear most of a +3 outfit (20+20+20+20+40=120) to get to unity level 3 and unlock this achievement. That means buying three ranks of Embroidered Investments and finding the relevant blueprints. I don't recommend rushing this, as some of the other items for sale at the Soul Shop are much more helpful. The blueprints that drop from chests are based on the difficulty of the room/zone you're in and so you will have to do multiple New Game Plus runs to bump up the difficulty enough to find the appropriate blueprints.

Note that some of the unity bonuses are excellent and are almost certainly more important than the stats you get from the equipment. For instance, the Sanguine armor set has poor stat bonuses, but its unity bonus gives you additional Lifesteal runes, which heal you upon killing an enemy, based on your strength stat. I suggest upgrading your equipment as you go to stay flexible. That way, you can mix and match and benefit from multiple bonuses, or specialize in one based on what you intend to do (Lifesteal runes are mostly useless in a boss fight for example).

TLDR: This achievement will take a while. Invest in the Embroidered Investments option at the Soul Shop while killing bosses and continue doing New Game Pluses. You'll eventually find +3 blueprints in chests. Buy and wear most of a +3 outfit to unlock the achievement. I don't recommend rushing this because it will make things harder for you.