Ok, so basiacally, when in a guild group (idk if you ahve to be) we were getting 87 guild rep per boss... on the 3rd boss in the lost city of tolvir, we had to replace the tank because he had to go.. brough a new one in (inexperienced) and we wiped 3 times on the boss. on the 4th attempt, we downed him and instead of recieving one set of guild rep, i got 4x the amount. it was pretty damn awesome and im pretty sure if you just wipe on the boss a few times, say 9, and down him the 10th, you will get however much guild rep you get per boss (mine was 87) multiplied by how many times you have been in combat with him. Hope this helps, seeing how big of a bitch guild rep is to grind out!
edit: tried it on next boss and it worked agin so added another pic
one more note, it might affect the guild xp you get from bosses aswell.