Shooting: In the level 1 try not to blow locks because it's not a quiet affair. Shoot them instead. Also use car windows as shield: shoot through them!
To unlock the artwork, spend the corresponding amount of Intel points at the "Special Features" menu:
Part 1: 10 Points
Part 2: 10 Points
Part 3: 10 Points
Part 4: 10 Points
To unlock weapons, spend the indicated amount of Intel points at the "Bonus" menu:
AS9: 60 Points
GL69A: 100 Points
M249SPW: 80 Points
MP7SD: 40 Points
MTAR-21: 70 Points
P90TR: 80 Points
SPAS 15: 40 Points
SR-4 CQB: 100 Points
U100MK3: 60 Points
UMP-9: 50 Points
USAS 12: 70 Points
Successfully complete the task in P.E.C. mode to get the corresponding medal:
Mercenary: collective 100 kills
Purple Heart: killed a collective 100 times