I've seen a lot of guides, including on the DS2 wikis, for killing the NPC Mad Warrior phantom to level up the Bell Keepers covenant, without having to rely on being summoned for PvP invasions to fight other players. Maybe you suck at PvP as I do, maybe you dislike risking bumping into cheaters, or maybe you just can't be arsed to wait for summons. These guides usually involve killing every mob in the area and a Faith stat investment to your build in order to utilise the Unveil miracle, that will move towards the enemy, should he have spawned after resting at the bonfire. However, on PC there exists an alternative approach to saving time running to his spawn area and warping back to try again.

Simply open Task Manager in Windows. There is no set RAM usage amount (can depend on what apps you have open ofc), but if you leave TM open on a second window and/or screen, you can see if he has been spawned in. If the RAM usage stays the same or decreases, he's not there; but if it jumps upwards then it's almost certain that he has spawned (if no other mobs have been killed and hence respawned themselves).

Simply rest at the Belfry Sol bonfire in Iron Keep repeatedly until you see RAM usage climb by a significant amount (typically around 10mb), climb the ladder and kill the bugger. Sorted.