The Enforcer skill tree is for players who like to shoot first and be stealthy later. Skills here focus on gunplay. Good ones to unlock here include...
...Underdog. This grants a damage bonuses when fighting three or more enemies. This is a common situation, so bonus damage is nice.
...Ammunition Specialist. This gives you a second Ammo Bag. When the party is split, having two stations for replenishing ammo is quite convenient.
...Pumping Iron. Unlocking, then acing, this skill boosts melee damage to all enemies. Turning a corner and bumping into a SWAT officer is a good time to throw a punch, so be prepared.
...Fully Loaded. This increases the ammo capacity for all weapons. Pretty great, eh?
...Portable Saw. This unlocks the OVE9000 for use. This handheld saw gets through doors faster than picking locks, so it's a great upgrade.
...Iron Man. You can't wear the Improved Combined Tactical Vest without unlocking this skill, simple as that.