To start the Level 20 Mercenary Life Quest, go to Placid Plain and speak to Stanford (X:250 Y:507). Before you see him, you should have 1 Vitamin A Pill handy as he will ask for one. After a cutscene, you and Stanford will fight together against three Shadow Assassins (level 24 Non-Elemental Mercenaries). Stanford must survive.
If Stanford dies while in plot a red cloud appears and steals all of your HP.