Andraste was burned at a pyre. Hessarian, her enemy, ended her suffering with his sword. But before that the flames pretty much did their job – she wasn’t really spared any suffering. Poor Andraste. :(
HOWEVER. I have a tin foil hat theory that Andraste was actually possessed by something (something happened to her and her sister in their youth around the same time Dumat was slain), and my theory is that when Hessarian slew her at the pyre it actually transferred in to him. This is why, when his blade touched her heart, he heard the voice of the Maker and continued her work.
OOC fun fact: Andraste is also the name of an Icenic war goddess.