Same as “Hunter’s United”, save you have to complete one hundred quests while playing online multiplayer. The sooner you start this, the better, and it’s obviously easier if you have a group of reliable, competent friends to play with. If not, your best bet is to either be a saint and help out weaker hunters who send an SOS Flare out (be a pal… and take on quests that are easier and quicker for you to complete) or, failing that, by repeatedly joining games to hunt monsters who drop materials you need. Since the entire end-game gameplay loop involves hunting Tempered Elder Dragons… that’s as good a target as any, especially since it’ll help you make progress towards numerous other trophies.