When using the Pathfinder class, the only way to fill your special ability, is to find tactical caches in the open world. The tactical caches contain random items, including the uplink protocol device.
Once you find these devices, you can use them to charge the Pathfinder's special ability, which allows you to hack into an Azrael drone and use it against the enemy.
With the Pathfinder as any class you are using for tactics -- on pc it's selecting the top middle item (like you would for grenades or emp's) and press the ';g' key. It than gives some info and your blue is ready to use with the 't' key on pc. So, you simply find a tactical cache (which you see thru your drone) and select the top middle special tactic ability (like gas grenade for Assault), and you are ready to let the special ability rip.
You don't have to wait for the blue to fill up -- kind of neat that way. I have used it a lot and am just trying to increase the level.