Greatsword is a very simple weapon. There are no magic combos that will make you amazing. It's simply about being patient and not getting too greedy. If you go in swinging, when a monster is fully mobile, you're asking to get hurt. Run around with weapon sheathed, observe, learn attack patterns. When you a find a decent opening, go in with an overhead. If the opening is large enough, charge it. If not, just get in the single swing and roll the hell away. Don't ever have your weapon out when it's not necessary. As soon as you're done attacking, you should be sheathing it again so you can run around and wait for more openings.
That said, there is one move you should definitely utilize: the super slam and followup super swing. I don't know if you know all the ways to access it, so I'll just list them here:
- A > A (keep hitting A after a basic roundslash). This one is not recommended because it requires you do to the roundslash first, which takes more time.
- Kick > X (sideslap) > Hold back on stick + X. This one is decent. If you don't want to hit the touchscreen for the kick, you can also kick by hitting R + X. Generally, use this one if you're already close to the monster.
- Dodge > X (sideslap) > Hold back on stick + X. This is my favorite; it allows you to go straight from a dodge into the sideslap, making it super fast to pull off and allowing you to close distance at the same time.
You'll know you're doing the super overhead right it right when your hunter swings his sword sort of diagonally as opposed to perfectly vertical.
The biggest perk of the super slam as opposed to regular charge (apart from it just doing more damage) is that you can hit X again after it to do a followup super swing which does just as much damage as the slam, effectively doubling(!) the punishment.
Generally, you should be using charge attacks and super slams most of the time. If you have time to follow up a basic overhead swing with a roundslash, you have enough time to pull off a full charge, which does far more damage.