Before you try to destroy next air defense element it is worth to get a Panzerfaust (M4,6). This weapon is also useful in destroying optional tanks. From the place where you killed the harbor master (or from the end of the hallway in the wooden building) go north to a bigger warehouse. Right behind the door on the right there can be a soldier. After hiding his body use the stairs to go up and to the adjacent building, still going north.
It is recommended to get rid of two guards that frequently look through the openings at the main square. However, hide their bodies inside the building - there is a sniper outside that can raise the alarm. You can eliminate the sniper from this place (M4,3) - he is an easy target even if you don't use suppressed ammo.
Your target, the armory, is on the ground level. You just have to go down the stairs and you are there. But be cautious because inside you can find at least one guard and the door to the bigger warehouse are wide open so someone can notice you. Near these doors there is a Panzerfaust (you must take it!) and ammo.