THEY CUT SHIT UP YO.
For all real intents and purposes, Great Swords are a double edged sword. Potential to be game breaking, but you need the SKILL and preperation to make them into that. A very finesse weapon, but what weapon isn't in monster hunter?
BASICS OF GS BECAUSE I AM TIRED OF INEPT FAGGOTS (Emiya)
All GS attacks use upwards of 80% of your raw power, ranking from highest to lowest:
If you hold down the downswing button (Default left attack,) you can charge up that attack to deal out additional damage.
The best combo for you to use is horizontal to downswing, because if you upswing you will usually wind up launching your teammates away and we will hate you forever (Emiya.)
Learn to cancel your post-attack animations by rolling. You can roll forward or to either side after you finish the attack, it saves the time it takes for your fat ass to pick up the sword and ready it. A good trick to use when fighting wyverns is to stand under where they land and time a charge hit so that it hits them as they land, this has a chance of knocking them over or disrupting their balance for a short moment.
One problem I see with alot of GS users is that they get really greedy with combos because combos can go indefinitely. Your ability to live is more important than how much damage you output. If you stick around for an extended combo and get your shit rocked and fail the mission, that falls entirely at your feet. Learn to roll out of the way.