Remove the 2-drain plug under each head.
You should get about 6/9 (about 18 total) liters this way, to get the rest,
detach one end of both of the 2 big cooling plastic tube (Coolant
distributor side is easier / rear side) and pull those close to the ground.

Just to be sure, unplug the one of the heater coolant line that goes to the
front, the best place is at the junction ("T" parts) of the rear radiator.

Open all bleeders and the expansion tank cap.

I would say that this way you would get about 80/90 % out, forget about
100% or pull the engine apart

Lol, Ben
Coolant system flow
http://pages.infinit.net/engine/cooling_system_flow.htm

Bleeding
http://www3.sympatico.ca/huotx/bleeding.htm