This trophy is titled "I'm Faster Than Anybody!" and it's for earning 5 first hits in a row while playing online in either player or ranked matches. First hits can be tricky to earn legitimately, given the fact the few randoms on the game know it fairly well. All I could suggest in this regard is to use either a character with long reach such as Hakumen and his forward + B (Circle) slash or a quick character like Taokaka. Regardless, getting this legitimately will be quite the task and I'd strongly suggest finding a partner.
If earning this trophy with a partner, it's easiest to do this in player matches given the fact you won't have to find each other. Invite your friend to a private game and host a match with as many rounds as you'd like. I'd personally suggest one round matches to make faster progress towards the 100 played but that's just me. It should pop in the post game after your match in which the 5th round of quick strikes occurred, assuming they were ALL IN A ROW meaning your opponent never struck you first between these 5 rounds.