Once you are in third level pick up the sniper rifle, stick grenades,
keys and other ammo in the store room by entering into it from the top.
You can reach to top by using ladder on back side of building.
After you have collected the ammo again go on the roof. Lay down behind
the wooden block and first hit the searchlight guard. By doing this you
won't come in notice of the officer and other guards. If by any chance
you are noticed select the Lugar Pistol and blow up the guard in one
shot by hitting them on head.
 
If you some how go on low Ammo try searching it near dead guards. If an
enemy is behind an oil barrel hit the oil barrel which will kill the
guards behind it.
To Kill Guards Easily In Segvicks Level
Once you have got the vehicle run your vehicle  over the guards in the
way. This will save you from damage and will kill opponents easily.