complete summoning is a perfect summoning that uses the life force of the summoner to call forth Fenrir himself rather then a portion of his power in standard avatar form. The very act of it is a forceful summoning of Fenrir, something Fenrir really doesn't want to involve himself in.
The pact he is referring to can either mean the one just created, or can be the original pact created by the kuluu to aid the people long ago. I tend to lean towards the former tho, at least that was how I interpreted it.
Either case Fenrir can be picky what he wants to help with, he's seen the future knows what going to happen and is content to let time run its course. Altho he is impressed with people who will stand up to their fates in a attempt to change it,one of the reasons he honored the first pact way back in zilart times to fight the zilart.