Garfazz can be found south of Grendich Courthouse in a ruined temple that the Charr have taken. It is easier to get there starting from Grendich Courthouse. If you start from there you will only have to fight some gargoyles and the usual Charr. Once you get up to Garfazz Bloodfang you will find a large group of Charr around him. It will be best if you have at least one healer and a full group. Kill the healer Charr first and the rest is up to you. It is possible to pull Garfazz out of the group, or simply find him alone, then you can just kill him and leave the rest of the mob alone.