To get this achievement you simply have to put specialization points into each node on a singular specialization tree so it will take some grinding.
edit: thanks to sneakysnabbalot for reminding me, some nodes will cost you 10 points such as the specialist pistols.
Ways to earn specialization points:
Completing Occupied missions
Completing certain projects
Completing mission replays gives you points based off the difficulty
There are other activities as well that give you points so be sure to read the rewards.
Some nodes cost 5 points and some will cost 3. You do not have to max out the nodes that have upgrades available. For example the demolition specialization has a node that increases explosive damage with 3 tiers you only have to invest points into one tier. Same goes for nodes that require 5 points.