When at the Secondary Objective, either on Defense or Assault, stay close to the stairs if possible. If you are out numbered or need to reload and are underfire go up the stairs while reloading. Hopefully the opfor won't see you go up the stairs and will think you have taken a defensive position and will throw grenades. If so run to the 2nd Floor office that is across from the middle entrance and jump down and surprise them, or go to the other room and climb out onto the rack and kill them.
When on assault, run outside and lay prone under one of the trucks. From there, aim and shoot at any OpFor that you see go by the upstairs window. Defense usually doesn't take the time to aim down below the trucks.. and if they do... they are exposed for a VERY long time.
Also, please allow the 203er to get outside as soon as possible so that they can 203 the window. 203er, please don't waste time by climbing up the ladder and camping assault north spawn, get outside and provide cover for your team and stay outside until you are out of ammo for the 203. SAW should try to rush to S/R and get up the stairs and mow down the opfor trying to cook a grenade to throw into the stairs.
Defending in Weapons Cache is pretty easy as long as everyone isn't rushing off to meet the assaulters. There are many great places to camp and wait for the opfor to walk right by without noticing you. If you rush to the 2nd Floor Hallway, don't all crowd into one room, one grenade will take out the whole team, sometimes it is a cooked grenade of a friendly that kills everyone. If one person just crouches in the doorway and waits usually you will get a few easy kills from someone peeking around a corner, or just running out without looking. Defending the objectives by finding a good camping spot is always fun, but most people know all the good spots and will blind grenade them.
If you hear an opfor taking the Secondary Weapons Cache and are on the Second Floor, it is possible to shoot them if the are crouched behind the crate taking the objective. This is a pretty easy shot, all you have to do is stand behind the trash can in the room with access to the rack, and you can see their head. Usually will result in a headshot.
North: If you are in Charlie spawn, then north is where you go. The squad leader should rush down north stairs, and then head to Loading warehouse. The AR should head down the ladder, the rush out and try to take down the opposition as the come down the hallway. Remember to take cover behind the boxes halfway. it is hard for an enemy to shoot you or throw a grenade without exposing themselves for too long.