When you face a quest Boss you have no chance of defeating, restart the game with other Covenants set to a higher level than your own (preferably as high a level as you can use and still use the relationship trick). Once done, trade equal amount of gold with a Covenant until your relationship is high enough to start an alliance. Then, select "Adventure Together". Go to the quest Boss and stay back while your over-leveled Covenant partner kills it. Next, restart the game at the normal difficulty setting. When you get to the same quest, you can select "Skip" and continue with the next quest.