I'm not actually sure how this works. There have been times that I had 300 credits available before a boss fight and times that I only had 10 credits available before a boss fight. Each fight I would skip the cooldown 3 to 4 times which would equate to 60 or so guild credits. But when I would look at my credits after the boss fight they would still be the same. so I am not exactly sure where the credits come from.