Oh, sorry man, I hope I didn't sound like an ♥♥♥ or anything. Monsters in the online quests are roughly 2x stronger than the ones offline. So a normal Jaggi that can be killed in one swing will take 2, and so on. It's not a big deal till you fight tough monsters like rathian and such cause their attacks deal hella hard damage. I advise an armor rating of at least 100 to fight online "strong" monsters.
That being said, you can complete the majority of 1 and 2 star online quests alone if you've beaten the single player campaign up to the 4 star missions and have decent gear. In past MH games you could take on the online alone and it was just really tough, I haven't played enough online for tri yet to make that judgement.