In 2007 the Buick Lucerne was equipped with 2 engine options. 3.8 Liter OHV V6 and the Northstar 4.6 Liter V8.
Was the cooling system recently opened? What was serviced?
The 3.8 has a 7mm bleeder screw on the top of the thermostat housing. A funnel in the radiator is helpful to bleed the air. You will notice the upper radiator hose curves to a higher elevation than the bleeder screw. Manipulation of the hose by hand may be needed to get the air out. I recommend and use a cooling system funnel to help. It minimizes mess as well. ( https://smile.amazon.com/Lisle-24680-Spill-Free-Funnel/dp/B00A6AS6LY/ref=sr_1_1?ie=UTF8&qid=1488855489&sr=8-1&keywords=lisle+cooling+system+funnel
) There are other brands available as well.
I do not recall if the Northstar V8 has a bleeder on all models, but some do have a similar bleeder screw. Be sure the system is filled initially and manipulate the hoses up and down as well as squeeze them to get air moving. The Northstar water pump is on the back side of the engine driven by the camshaft. The upper hose runs along the front of the engine bay and connects to the radiator at the top on the passenger side of the engine bay.
For both vehicles, fill as much as possible cold. Start and idle. As the temperature increases, I recommend occasionally revving the engine to between 1500 and 2000 RPM to fill heater core and move coolant with a little more force.