The bottom line is really quite simple - you just have to play and practice and that's it.
When I started off with FH, I got my arse handed to me on a plate many times, by everyone. The first totally OP character was Centurion who sliced through everyone like a hot knife through butter and believe it or not, was the reason I stopped playing FH for a while. That was back a couple or so years back, while playing on PC. I moved to PS4, started again, got to a high rep and sold the PS4. Now I'm back on the PC. i found that really you just have to keep to good zoning. It's like any martial art or video game. Spacing is vital to winning games. But with Centurion his kick is easy to dodge and he's going to try to grab you a hell of a lot. Once you guardbreak his guardbreak and dodge his kick you take away a vital part of his arsenal.
My game is to take a character and play them up to Reputation 10. Then rinse and repeat with the rest. You not only get a feel of the character, but learn their attack strings, feints and guardbreaks.
Also, while playing as Assassins is fun, they also have the lowest health pool in the game and require skill to play. Start off with heavy and Hybrid classes as they have the most health. Then move to Vanguard and assassin.
I'm now Rep 156 and I've taken 14 characters to Rep 10, some a little higher. I now have an overall good understanding of the knight, viking and samurai factions. I'm still not fully aware against, the Wu Lin, although I did take Jiang Jun to about Rep 15 on the PS4 and I've taken Nuxia to Rep 10 on the PC. Tiandi and Shaolin give me problems and to a point Zhanhu.
And then, of course, you'e going to have hard matchups. Jormungandr gave me real problems, but now I'm beginning to understand him more, looking for the grab, dodging the punch after the blocked hit and attacking.
It's simply a case of putting in the time and learning how the enemy works. Remember Sun Tsu - "know thy enemy, know thyself."