This achievement requires you to craft five weapons at the armory which have the highest rarity level. In Iceborne, this is Rarity 12.
At the armory, select one of your weapons that has upgrades available. The panel below the weapon grid will display the name and cost of the next weapon in that particular upgrade tree if you have the required materials on hand, or it will display ??? for the weapon name and cost if you do not. If a weapons has no available upgrades, its icon will be darkened and the message "Maximum upgrade reached" will appear below the weapon grid.
The weapons which can be upgraded to Rarity 12 will have an icon colored white with a sparkling effect at the end of their tree, all the way to the right of the upgrade display. Most players will have a weapon or two that was fully upgraded in base MHW, so the easiest way to approach this achievement will be to start with those weapons first. However, if you did not craft many weapons in base MHW, you may find yourself needing to run a LR/HR hunt or two to craft a weapon early in an upgrade tree.
It would be exceedingly long and a waste of space to try and breakdown which monsters should be targeted over others, because the makeup and maximum rarity level of the upgrade trees varies from weapon to weapon. For example, Zinogre. His switch axe tree maxes out at R11, while his hammer tree maxes out at R12. Conversely, Namielle has a R12 switch axe, but only a R11 hammer. A good rule of thumb, however, is to simply look at the difficulty of the monster. The more difficult it is to defeat, the more likely it will be that it has a R12 weapon.
A final note: Several of the R12 weapons require monster materials which are quite rare - in general, it is the material at the bottom of the "Required Materials" list for whichever weapon/tree you are looking at. However, after you have defeated a monster once, you can use the Elder Melder to meld most of these rare mats using Celestial Wyvern Prints obtained from the Steamworks.