Zoos accredited by organizations like the Association of Zoos and Aquariums have weapons teams trained to use deadly force to prevent death or serious harm:
Although the procedures followed by the ‘weapons teams’ are standardized, the firearms used appear to be chosen by the individual zoos and/or the leader of each team. Open source information points to a combination of 12 gauge shotguns and high-powered rifles being on hand at most major zoos.
From a story in the St Petersburg Times:
The team armed themselves with four guns from a locked cabinet kept in the general curator’s office. Salisbury carried a 12-gauge shotgun. The remaining staff carried two .375 rifles and a 30-06 rifle.
Zoo employees also train and qualify with local and state law enforcement agencies.
From a story in the Pittsburgh Tribune:
Pittsburgh Public Safety Director Stephen A. Bucar said police officers and zoo workers went through training immediately after the incident Nov. 4, 2012, when 2-year-old Maddox Derkosh was killed. Bucar said police don’t carry weaponry needed to bring down a large animal in the event of a similar incident. They don’t know enough about animal behavior to shoot an animal, he said.
Always make sure that firearms are on safety and handled with extreme caution. The use of a killing weapon must always be tempered by the potential to endanger human life.
Whenever possible, the shooter should stay in a vehicle when approaching the animal.
Never run after the animal. It’s certain that you can’t outrun it. You will be out of breath, which will not allow you to have a steady hand.
Make sure you have a good clean shot. Be aware of what is in front and behind your target.
If you must shoot, shoot to kill. If you do not feel you are capable of doing this, relinquish the responsibility to another qualified shooter (if one is available)
If you haven’t seen the raw footage: