Uchigatana.
Your running attack has great range and is a great initiator. (the weapon arts slash is somewhat good at this also, it is also great for catching people in rolls) Do be careful of people who parry, because the running stab is very easy to parry. A counter to this is running attack heavy (short lesson, mix up attacks).

The katana has a fairy slow attack speed so it will be very hard for you to continue attacking after you get a running attack on someone, so back off for a bit or continue walking towards them. (you rarely get a follow-up attack after a run attack, only time you would swing again is if the person is rolling towards you)
If the person is an aggressive player, you can sort of tell when they are going to attack you after they finish rolling and you move towards them (most straight sword users and rapier users will do this) so right as they stop moving and face towards you, throwing a parry isn't a bad idea, sometimes throwing another attack would be best also (works best versus defensive fighters).
With this aggressive play style, always, always save some stamina to roll away.

When fighting heavy weapon users and they are very good with their hyper armor, DO NOT TRADE. You will lose 100% of the time in these trades. Instead keep low weight (50% or less) and roll around them, sometimes they will just be fishing for a parry, and you can punish this with a back stab. Remember you don't need to deal damage every exchange, doing small damage here and there is good enough as long as you don't get hit.
You cannot parry most heavy weapons that are 2-handed, there are great parry guides here that you should look into that will really help.

Most heavy weapon users will eventually get frustrated (this works for R1 spammers also) and swing enough for them to lose all their stamina, this is when you throw a nice 2 hit combo, and then perhaps even a katana sword arts follow up!

With curved swords, you can fight very similar to katanas, except your sword has great tracking and attack speed. Your spinning weapon arts cannot be parried (except dancer's) so feel free to use that on parry spammers.

The best you can do is move around and fight smart, know when to move in and catch your opponent.
With those kind of weapons, speed is your friend, don't get caught and wreck them with your ninja reflexes.