I found another article about the 6.69 Secret Quest---Killing Phenix, here is the guide on How to Kill Phenix in Dota 6.69
http://www.pubarticles.com/article-dota-6-69-quest-guide-killing-phoenix-in-dota-6-69-1286933491.html
Hoping it can help you finish that secret mission, but honestly Phenix is relly hard to kill, you got be patient to this mission.