Brotato PS4 & PS5

Weapons can be combined together to upgrade them up tiers this achievement requires 3 red weapons at once. Combining weapons is dont by having 2 of the same weapons or buying a weapon of the same rarity from the shop with a full weapon inventory.

Alternatively having a high enough luck can increase the chance of Red weapons spawning in the shop

Tier 1 - grey
Tier 2 - Blue
Tier 3 - Purple
Tier 4 - Red (MAX TIER)

You must have 3 Red weapons to have this achievement

To unlock this achievement, you just need to have at least three weapons on your character that are Tier IV. The tier levels of these weapons are set by the background color as follows:

Tier I - Black
Tier II - Blue
Tier III - Purple
Tier IV - Red

Weapons can be upgraded by merging two of the same type that are on the same tier. Alternatively, who you pick as your character can affect your odds of getting certain weapons you want at higher frequencies when you go to the shop after every wave and with rerolls for new gear. I discovered this from picking Ghost as my character, he has three Ethereal weapons you can pick from before starting your run (Ghost Scepter, Ghost Flint, and Ghost Axe) and they all come up in the shop rather often. That helped me with upgrading my whole weapon set that unlocked this achievement, and as such Ghost is a character that's well recommended if you want to unlock this one yourself.