australian convicted drug smugglers myuran sukumaran and andrew chan are expected to be executed in coming hours .
celebrated artist ben quilty became their mentor and friend in 2002 .
he has campaigned tirelessly for clemency , leading the ` mercy ' campaign .
hours before their execution he penned a message to indonesian president .
he has described how the men will be a pillar of strenght for the others on death row until the last moment the guns are fires .